Rdtsc vs rdtscp. Works on both Windows and Linux.
Rdtsc vs rdtscp /// /// The processor monotonically increments the time-stamp counter MSR /// every clock cycle and resets it to 0 whenever the processor is /// reset. and that the possible I am trying to write a program to measure context switch. On AMD, the change is either on-par with the current LFENCE-prefixed RDTSC and some are slightly better with RDTSCP. A microbenchmark on Intel shows that the change is on-par. 2. Processor time - how many cycles? this is related to wall clock time by processor Latency of RDTSC and RDTSCP instructions on Intel CPUs ( VTR-028 ) This bit is 1 if the instruction is supported, and 0 otherwise. So RDTSC is actually the wrong instruction, you need to use RDTSCP, and check that the CPU core used was the same across both measurements. I have gone through this Intel's manual about rdtsc + rdtscp instructions. There is always some indirect information leakage because there are no Intel processors that have no shared resources!. Since RDTSC has no inputs, the CPU itself may reorder the RDTSC instruction to come before the code you are trying to measure. 生成 rdtsc 指令，该指令将返回处理器时间戳。 处理器时间戳记录自上次重置以来的时钟周期数。 语法 unsigned __int64 __rdtsc(); 返回值. RDTSC is the most precise. If this should occur and Secure TSC is enabled, guest execution should be terminated as the guest cannot rely on the TSC value provided by the hypervisor. So this is now pretty much a duplicate of that canonical Q&A. I don't think GCC "knows" which CPUs support it, and definitely doesn't define any macros with -march=native rdtsc instruction. This can foul up any measurements you might make with it. Automate any workflow Packages. If software requires RDTSC to be executed only after all previous instructions have completed locally, it can either use RDTSCP View the full free MOOC at https://ost2. But rdtsc is non-serializing, so using it alone will not prevent the CPU from reordering it. Read Time Stamp Counter instruction. To fix this problem, we can add a CPUID right after the first rdtscp, which unfortunately defeats the Difference between rdtscp, rdtsc : memory and cpuid / rdtsc? 63 How to get the CPU cycle count in x86_64 from C++? 19 Why should I use 'rdtsc' differently on x86 and x86_x64? 2 How to detect rdtscp support in Visual C++? 21 RDTSCP versus RDTSC + CPUID. In other words there is no effect on output of rdtsc when I change the frequency Can anyone please tell I see. What I understood is that they use both -- one to start and one to end the measurement, since one combination of cpuid and one prevents instructions to be executed before, and another combination prevents instructions to be executed after, therefore reducing The RDTSCP instruction is not a serializing instruction, but it does wait until all previous instructions have executed and all previous loads are globally visible. In x86-64 mode, RDTSC also clears the highest 32 bits of RAX and RDX. A 64-bit unsigned integer representing a tick count. We have some candidates here: The rdtscp instruction, which is available all recent X86 platforms. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company RDTSC actually does something like that – “The RDTSCP instruction waits until all previous instructions have been executed before reading the counter. Most source and header files maintain original copyright notices on top, since I just adjusted function names and a few things here and there. (The manual says L’intrinsèque __rdtscp génère l’instruction rdtscp. In my post above, I made reference to the minimum latency for consecutive RDTSC and RDTSCP instructions, but I did not include the details. , cat /proc/cpuinfo) and then adjusting our TL;DR. My main objective is to avoid the need to perform system call every time I want to know the system time. Some of my tests used a single RDTSCP instruction per loop iteration, so they will always have the same alignment, while other tests unrolled the loop to see if loop control overhead made a difference. Manage code changes Discussions. According to Microsoft: We strongly discourage using the RDTSC or RDTSCP processor instruction to directly query the TSC because you won't get reliable results on some versions of Windows, across live migrations of virtual machines, and on hardware systems Internally, this is using rdtsc or rdtscp for the fine-grained portion of the time-keeping, plus adjustments to keep this in sync with wall-clock time (depending on the clock you choose) and a multiplication to convert from whatever units rdtsc has on your platform to nanoseconds. More than 100 million people use GitHub to discover, fork, and contribute to over 420 million projects. Using RDTSC on the same section of code can often produce very different results. To correctly use RDTSC, you are faced with a Heisenberg problem "the act of measuring influences the thing that is being measured". High-resolution cycle counter. Monitoring process id 0x490 for RDTSC/RDTSCP instruction execution and run 3 nops whenever the event condition is triggered and runs 3 nops whenever the event is triggered. I mean the rdtsc() is fixed for 3Ghz and does not change with frequency. unsigned __int64 __rdtsc (); Return value. Difference between rdtscp, rdtsc : memory and cpuid / rdtsc? 63 How to get the CPU cycle count in x86_64 from C++? 19 Why should I use 'rdtsc' differently on x86 and x86_x64? 2 How to detect rdtscp support in Visual C++? 21 RDTSCP versus RDTSC + CPUID. Host and manage packages Security. To covert RDTSC clock ticks to time, need to divide it with CPU clock frequency in GHz. Variability of the CPU's frequency. Is there any difference in between (rdtsc + lfence + rdtsc) and (rdtsc + rdtscp) in measuring execution time? As far as I know, the main difference in runtime ordering in a processor with respect to rdtsc and rdtscp instruction is that whether the execution waits until all previous instructions are executed RDTSC can return inconsistent results for a number of reasons:. Lately i've been playing around with hypervisors and rdtsc / rdtscp trapping and offsetting. Find and fix vulnerabilities Actions. Plan and track work Python wrapper around the RDTSCP instruction to get the cycle counter on X86 - Roguelazer/rdtsc. 3,239 2 2 gold badges 18 18 silver badges I can't say for certain what exactly is wrong with your code, but you're doing quite a bit of unnecessary work for such a simple instruction. It doesn't stop you from using __builtin_ia32_rdtscp() / __rdtscp() (or rdpmc) the way it does for most other intrinsics, when compiling for -march=core2 or generic. If two logical processors are in the same hardware SMP, then they are sharing the coherence fabric (via the My Final update of RDTSC is using RDTSCP which more efficiently now we back to use SetThreadAffinityMask in core 0 since without its can cause drift time and loops in cores reduce the freq to 1 microsecond more efficiently Enjoy the best timer RDTSC + CPUID (in reverse order, here) to flush the pipeline, and incurred up to a 60x overhead (!) on a Virtual Kernel-level emulation of rdtsc for Mac OS X. If I call clock_gettime() in quick succession, the length of time goes from about 45k clock cycles down to 500 PoC that measures how long it takes the CPU to execute the CPUID instruction and reports if it suspects a VM. TSC/ rdtsc allow to measure time in an Update, about aux: The RDTSCP instruction returns the TSC (in two registers), and the Processor ID (aux) in a 3rd register (unlike the RDTSC instruction which only returns the TSC). Also are you sure it's synchronized across sockets? rdtscp also gives a 'processor ID' which only makes sense to I've asked this question plenty of times wrt rdtsc. The following items may guide software seeking to order executions of RDTSC: • If software rdtscp is more than 4 uops, and its one-way serialization barrier effect means it shouldn't be possible for two rdtscp instructions to execute in the same cycle as each other. Copy HyperDbg >! tsc pid 490 code {90 90 90} condition {90 90 90} Or if you want to use assembly codes directly, you For even higher resolution, rdtsc may be on put the table. CPUID is also well-known as a representative instruction for which the required cycle count differs significantly between virtual and real machines; thus, it is useful for sandbox detection. Use. PowerPC provides similar capability. and that the possible Read this whitepaper and use its methods for reading the x86 cycle counter (using "rdtscp" if your CPU supports it, or "rdtsc" otherwise). 86. That's not a meaningful microbenchmark even on bare metal. We’ll pair : that with a CPUID instruction which acts as a memory barrier, resulting in this: */ static The "RDTSC()" macro invokes the rdtsc assembly language instruction, which basically transfers the contents of the CPU's internal clock counter register to a 64-bit variable. The operating system can program this value to be the ID of the CPU, and a sufficient amount of information is emitted in a single instruction. Get CPU cycle count? for that and more about RDTSC. It counts at a fixed frequency regardless of turbo / power-saving, so if you want uops-per-clock analysis, use performance counters. Ideally our benchmarking approach provides the following: Low variance for instructions involved in RDTSC instructions. The TSC is a 64-bit register on x86 processors. I tried (StopWatch), but it gives different time in each run, because of 但是，rdtscp仅在较新的 CPU 上可用。所以在这种情况下，我们必须使用rdtsc. The low resolution is no surprise since the value is often derived by shifting off the 10 low bits of the CPU's time stamp counter (TSC), after adding a value that reflects the cumulative sleep/hibernation SVM provides conditional trapping of RDTSC, RDMSR, WRMSR and RDTSCP instructions, which is enough for full virtualization of TSC in any manner. Gabe. Automate any workflow As rdtsc is a lot more suitable for performance measurements than QueryPerformanceCounter because of its much lower overhead, I would like to use it whenever possible. Instead, we encourage you to use QPC to leverage the For many years x86 CPUs supported the rdtsc instruction, which reads the "time stamp counter" of the current CPU. If I call clock_gettime() in quick succession, the length of time goes from about 45k clock cycles down to 500 The RDTSC spoof is detectable if you pass in the host invariant TSC so you don't pass it in, which means if you do this, the guest only has access to emulated timers and interrupts because it has no idea that it's running inside a virtual machine. The comparison is done with the LFENCE-prefixed RDTSC (and not with the The cpuid instruction is a full barrier, preventing reordering in both directions, while rdtscp prevents reordering from above. notes and example codes on using rdtsc(p) instructions for benchmarking - xuwd1/rdtsc-notes View the full free MOOC at https://ost2. For profiling, use perf counters. But I think you should be using a serializing instruction (like CPUID) before the first RDTSC. youtube. Instant dev environments Issues. Skip to content . RDTSC instructions. rdtsc is an instruction supported since Pentium class CPUs to read the current time stamp counter (TSC) which is incremented every CPU tick (1/CPU_HZ). If two logical processors are in the same hardware SMP, then they are sharing the coherence fabric (via the RDTSC_Calculator is a hook for The timestamp counter is a 64-bit register present on all x86 processors since Pentium. Issues with RDTSC usage. I tried to calculate the time of implementation “vmexit” and this time of implementation add to result of implementation function “rdtscp”. See RDTSC instructions. Also, remember that the TSC frequency is fixed, independent of the core clock, so in a tiny program, the CPU might still be at idle clock speeds while running the timed interval, so each core clock is many reference cycles. This instruction could be used to read the hardware counter like the rdtsc instruction, and while it is not a serializing instruction, it does wait until all previous instructions have executed and all I am using rdtsc for getting high-resolution, low-overhead timing, which is needed by my implementation. A cross platform timer. The following items may guide software seeking to order Its common to use RDTSC to get fine-grained timing information, where the overhead of a virtualization trap would be quite significant. You may need to configure constant_tsc_offset value, which is at 1000 by default. With respect to later instructions, rdtsc in the lfence/rdtsc sequence may be dispatched for execution simultaneously with later instructions. The kernel enables the mapping. RDTSC; RDTSCP; WRMSR; Volume 3 of the Intel Architectures Software Developer's Manual (document 325384-068) This is the "System Programming Guide" Chapter 18 "Performance Monitoring", and; Chapter 19 "Performance Monitoring Events", and; Volume 4 of the Intel Architectures Software Developer's Manual (document 335592-068) This contains RDTSC是什么 RDTSC是 “Read Time-Stamp Counter”的缩写，它是目前intel和AMD的CPU都普遍支持的一条CPU指令，该指令可以把当前处理器的时间戳加载到EDX:EAX寄存器供外部使用。RDTSC的优点 RDTSC是内置的CPU指令，而一般CPU单条指令运行也就需要几十个CPU cycles, 所以采用RDTSC指令可以在开销比较小的情况下获取程序 If changing from RDTSCP to RDTSC does not change the overall performance by a detectable amount, then it is likely that neither of them is actually a problem, and the VTune analysis is biased. RDTSCP versus RDTSC + CPUID. Note: If you can dig up a copy of Intel's System The value of this counter can be read through the RDTSC or RDTSCP machine instructions, providing very low access time and computational cost in the order of tens or hundreds of machine cycles, depending upon the processor. You save a byte by just calling rdtsc, which is the way recommended in that awesome Intel whitepaper – RDTSC locks the timing information that the application requests to the processor's cycle counter. So rdtscp reads 2 MSRs (so it has additional overhead) and it also waits for all instructions the RDTSC is internal to the CPU and increments every clock cycle of the processor, these days that's 1,000MHz or faster. While the first rdtscp guarantees that all the instructions before itself have been done, it cannot stop the CPU from executing some code from code_to_measure before rdtscp. Contribute to landonf/TrapRDTSC development by creating an account on GitHub. Also I tried to get the previous result of implementation “rdtsc” and add to it the number about 100 and this doesn’t rdtsc/rdtscp latency test; experiment to read TSC in one thread and pass results to another via memory - testing-laboratory/rdtscTest. The first time a line of data or code is brought into cache, it takes a large number of cycles to transfer it into memory. BTW: rdtsc is a terrible instruction: it needs to drain the instruction pipeline (IIRC you need to fetch CPUID too, to get reliable results), maybe even the TLB, and it sometimes even forces the cores to resynchronise their clocks. Plan and track work Code Review. This instruction waits until all previous instructions have executed and all previous loads are globally visible. This The RDTSC instruction is not a serializing instruction. Also, the rdtscp in your bench_start() is redundant due to the previous cpuid call. The following items may guide software seeking to order executions of RDTSC:</p> my patches for linux kernel to spoof rdtsc and make vm exit undetected - GitHub - WCharacter/RDTSC-KVM-Handler: my patches for linux kernel to spoof rdtsc and make vm exit undetected. I would appreciate any kind of help as I have been doing this for the first time. This section gives details on two different ways to handle cache Some new Intel processors have both RDTSCand RDTSCP instructions while most older processors have only RDTSC instruction. Perhaps the Created attachment 274869 Program to demonstrate latencies of lfence+rdtsc vs. A #VC exception will be generated if the RDTSC/RDTSCP instructions are being intercepted. Actally I am trying to measure the latency of a DVFS switch. , they don’t corrupt the measure) since there is a logical dependency between RDTSCP and the register edx and eax (RDTSCP is writing those registers and the CPU is obliged to wait for RDTSCP to be finished before executing the two “mov Replace that sequence with RDTSCP which is faster or on-par and gives the same guarantees. Ce bit est 1 si l’instruction est prise en charge, et 0 sinon. 表示滴答计数的 64 位无符号整数。 要求. Also are you sure it's synchronized across sockets? rdtscp also gives a 'processor ID' which only makes sense to It’s accessible via the RDTSC instruction, making it a popular choice for high-resolution timing on x86 platforms. RDTSCP. I am referring to this Intel white paper and also gone through other SO threads discussing the topic of using RDTSCP vs CPUID+RDTSC here and here. Merge an old patch from 2011 that supports VM_EXIT catching, then adapt it to the modern kernel module, flesh it out, build it and you’re good with most things (this will bypass pafish if the implementation you created is good enough and doesn’t take too long to return a RDTSC call) rdtscp=off most likely means that you wont exit on rdtscp. But you do have rdtscp itself in the measurement interval. Reload to refresh For my project I must use inline assembly instructions such as rdtsc to calculate the execution time of some C/C++ instructions. __rdtsc/__rdtscp for ARM Mac M1/M2? Hot Network Questions Disregard equation alignment in one line Does a USB-C male to USB-A female adapter draw I usually measure stuff with perf counters, not RDTSC. About; Products OverflowAI; Stack Overflow for Teams Where developers & technologists share private knowledge with Both instructions are guaranteed to be executed after RDTSCP [ Intel wrote RDTSC but meant RDTSCP] (i. Most common use is to have two RDTSC instructions with a small amount of code between them, taking the difference of the times as the elapsed time (number of cycles) for the code sequence. rdtsc and rdtscp. Microsoft Specific. Also, a single time in cycles isn't very meaningful for something as short as incrementing an integer; you need to measure throughput separately from latency. As I understand, on modern hardware, it measures wall clock time, so it's perfectly fine for benchmarking, as it's very low latency, but it can run afoul of pipelining, so for single measurements that must be accurate, you need to serialize it, usually with a call to cpuid (or alternatively, you can call rdtscp, which does the same). rdtscp Improved version, now compiles with '-std=gnu11 -Wall -Wextra -Werror' with no errors. The problem I am facing is that the latency of that operation or number of cycles taken by that operation remains the same even when I change the frequency of the processor core from 3 Ghz to 2 Ghz. 6 Detect if processor has RDTSCP at compile time my patches for linux kernel to spoof rdtsc and make vm exit undetected - RDTSC-KVM-Handler/README. Steven Hansen Steven Hansen. rdtscp doesn't prevent reordering, it only ensures all previous instructions have finished executing: The RDTSCP instruction waits until all previous instructions have been executed before reading the counter. The processor time stamp records the number of clock cycles since the last reset. - GitHub - TheDuchy/rdtsc-cpuid-vm-check: PoC that measures how long it takes the CPU to execute the CPUID instruction and reports if it suspects a VM. For very fine-grained measurements, CPU optimizations like superscaler issue and out-of-order execution can introduce unexpected variability, and this whitepaper discusses how to account for it. MFENCE is the wrong instruction for this; it's not guaranteed to serialize the instruction stream (but in practice it does on Skylake > On Mon, Jan 31, 2011 at 11:21:08AM +0100, Manuel Bouyer wrote: > > > I have a i586 CPU where rdmsr(MSR_TSC) doesn't work but rdtsc does. 6k 12 12 Latency of RDTSC and RDTSCP instructions on Intel CPUs ( VTR-028 ) If your test to detect reordering is assert(t2 > t1) then I believe you will test nothing. English Download and execute ! Português Download e I am using RDTSC and RDTSCP in NASM to measure machine cycles for various assembly language instructions to help in optimization. So in this case we have to use rdtsc. How can I detect reliably if the rdtsc is a constant rate counter or not? profiling; x86; 64-bit; rdtsc; Share. Take a look at Run Custom Code and how to create a condition for more information. However, it isn't a serializing RDTSC gets the number of cpu cycles since last reset, see wikipedia. ace, 13 years ago. In addition, SVM allows passing through the host TSC plus an additional offset field specified in the SVM control block. Skip to content. Automate any workflow Codespaces. I will compare them. Why in ns_per_rdtsc_tick() y Skip to content. It is a bit tricky to determine the "latency" of the RDTSC and RDTSCP instructions on recent processors for a couple of reasons: The TSC increments at the rate of the "nominal" processor frequency, not the instantaneous processor frequency. On some CPUs (especially certain older Opterons), the TSC isn't synchronized between cores. But since it's a 64-bit number, it's stored in EAX (low part) and EDX (high part), and if this code is ever used in a case where it is inlined, the compiler doesn't know that EDX is cpuid is very slow even when not in a VM (). I've finalised on using clock_gettime, but I did do sufficient research and testing with the RDTSC instruction as well. While coding in C/C++, how I can detect at compile time if the architecture being used have RDTSCP instruction or not?. . So I don't need to use inline asm code, which I am not familiar with. Low Overhead: Reading the TSC is faster than other timing functions. Ask Question Asked 13 years, 11 months ago. None Example TSCNS uses rdtsc instruction and simple arithmatic operations to implement a thread-safe clock with 1 ns precision, and is much faster and stable in terms of latency in less than 10 ns, comprising latency of rdtsc(4 ~ 7 ns depending on platforms) plus calculations in less than 1 ns. Opcode 0F 31 Flags affected. So we can use either of these two options to prevent reordering: 2: This is a call to cpuid and then rdtsc. ) And BTW, Hyperthreading can't be the issue for a single thread, it's superscalar / out-of-order single I added a section to Mysticial's answer on that question which explains how the asm works. Use a function provided by your library or OS. Monotonic: On most modern processors, the TSC is monotonic and RDTSCP. md at master · WCharacter/RDTSC-KVM-Handler. However, subsequent instructions may begin execution before the read operation is performed. cpuid is a serializing call. Syntax unsigned __int64 __rdtscp( unsigned int * The RDTSC and RDTSCP instructions provide low-overhead access to a counter that increments at a fixed frequency. Documentation says "We strongly discourage using the RDTSC or RDTSCP processor instruction to directly query the TSC because you won't get reliable results on some versions of Windows, across live migrations of virtual machines, and on hardware systems without invariant or tightly synchronized TSCs. Measured in seconds. RDTSC has The RDTSC instruction is not a serializing instruction. Navigation Menu Toggle navigation. It is not your case but it can be useful when you want to measure On older hardware, the frequency may actually change in between two RDTSC instructions, and even on newer hardware where it is fixed, it can be difficult to tell what frequency it runs at. Improve this question. The There are risks in using the rdtsc instruction for timing measurements, especially on older hardware. I have several question regarding the same (based on what I read on several online threads): Actually, that's not very good code at all. 5MHz. Counts the number of cycles since reset. Map; Sandbox Evasion; RDTSCP; Newer processors support a new instruction called RDTSCP which does the exact same thing as RDTSC, except that it does so serializing (meaning it waits for all instructions to execute before reading the counter. I know we can check this out manually by browsing CPU info (e. Ideally our benchmarking approach provides the following: Low variance for instructions involved in When in protected or virtual 8086 mode, the time stamp disable (TSD) flag in register CR4 restricts the use of the RDTSC instruction as follows. Instant dev environments Copilot. In this era of unprecedented digital connectivity, remote access to personal or professional computer systems is increasingly commonplace. When the TSD flag is clear, the RDTSC instruction can be executed at any privilege level; when the flag is set, the instruction can only be executed at privilege level 0. There are risks in using the rdtsc instruction for timing measurements, especially on older hardware. – //Increasing i took end-time cycles - RDTSC and RDTSCP have significant overhead compared to executing one instruction. If your test to detect reordering is assert(t2 > t1) then I believe you will test nothing. The alternative method to enable rte_rdtsc() for a high resolution wall clock counter is through the armv8 PMU subsystem. Which suggests the RDTSC instruction is not waiting for the first CPUID to complete before starting to read the TSC, is this correct? I understand that RDTSCP waits for the previous command to finish, hence replacing the RDTSC with a RDTSCP seems to work. Loads the current value of the processor's time-stamp counter into the EDX:EAX registers; It is commonly used as a timing defense (anti-debugging technique). What is the gcc cpu-type that includes support for RDTSCP? 1. It could well be that the kernel somehow disables this instruction, I don't know how, but for userspace execution the instruction can The rdtsc and rdtscp instructions are independent of misses. All measurments looks reliable. The RDTSCP instruction waits until all previous instructions have been executed before reading the counter. This bit is 1 if the instruction is supported, and 0 otherwise. From MSDN for rdtscp: To determine hardware support for this instruction, call the __cpuid intrinsic with InfoType=0x80000001 and check bit 27 of CPUInfo[3] (EDX). Write better code with AI Security. Comparing to the previous pattern, we replaced the first CPUID+rdtsc with rdtscp. This gives you a better estimate of the time "something" actually takes. Reload to refresh your session. Using RDTSC instruction that returns CPU TSC (Time Stamp Counter) - rdtsc. Don’t be deceived; this instruction isn’t cheap and measures in at 34 ticks on 我没理解错的话rdtsc不是应该需要加一个lfence或者mfence吗 . Stack Overflow. (Or sometimes you want lfence;rdtsc;lfence to start the clock, for extra repeatability at the cost of more overhead. I work on programming language profiler and I am looking for a timer solution for Windows with better than 100 ns resolution. Sign in Product Actions. On my Xeon E5-2680 rdtscp is much weaker than this, only draining the ROB but not the store buffer. For even higher resolution, rdtsc may be on put the table. Back-to-back rdtscp is not fast, 64 reference cycles sounds totally reasonable if you ran without warming up QueryPerformanceCounter and RDTSC. Note: You do not need to implement a kernel You mean the only difference between rdtsc and rdtscp is that rdtscp is serializing? I thought to get the TSC that's synchronized between cores you had to use rdtscp. I wrote some test code that used __rdtscp to time repeated calls to clock_gettime (the rdtscp calls went around a loop that called clock_gettime and added the results together, just so the compiler wouldn't optimize too much away). rdtscp produces a 64bit timestamp result (which is obviously the return value), and a 32bit IA32_TSC_AUX. Today we look at some techniques to get basic timing information from your running game. A Couple of ideas of time: Wall clock time - time as it passes in the real world. Write better code with AI As part of a benchmarking task, I was investigating the different mechanisms that can be used to measure elapsed time. TSC/ rdtsc allow to measure time in an Using RDTSC on the same section of code can often produce very different results. The problem is, the number of cycles do not change. Automate any workflow The rdtsc (Read Time-Stamp Counter) instruction is used to determine how many CPU ticks took place since the processor was reset. I check the rdtsc() before and after changing the frequency and subtract those 2 values to get the number of cycles. The 2 clocks are different. It sounds like you're already handling this by using sched_setaffinity-- good!; If the OS timer interrupt fires while your code is running, there'll be a delay introduced while it runs. StopWatch. In the above mentioned whitepaper, the method using CPUID+RDTSC is termed unreliable and also proven using the statistics. The normal advice is to use a CPUID instruction immediately before and/or Using rdtsc for timing extremely short instruction sequences is also problematic because of out-of-order execution. Share. The Performance counters are an external device which increments typicaly at about 3. The following items may guide software seeking to order executions of RDTSC:</p> Elapsed TSC cycles (using RDTSC or RDTSCP) Instructions — using the RDPMC instruction with counter number (1<<30)+0; Core Cycles not halted — using the RDPMC instruction with counter number (1<<30)+1; Reference Cycles not halted — using the RDPMC instruction with counter number (1<<30)+2; If you have the ability to program the general Remember that RDTSC counts reference cycles, not core clock cycles. Intrinsic Architecture; __rdtsc: x86, x64 : Header file <intrin. Sign in Product GitHub Copilot. com/channel/UCW_wN3GitbKr_GbV2K6 And at this point I have expected for them to work. Imagine we have a rdtsc2 that is exactly like rdtsc but writes my patches for linux kernel to spoof rdtsc and make vm exit undetected - GitHub - WCharacter/RDTSC-KVM-Handler: my patches for linux kernel to spoof rdtsc and make vm exit undetected. Created the Monday 14 September 2020. Imagine we have a rdtsc2 that is exactly like rdtsc but writes <p>The RDTSC instruction is not a serializing instruction. Syntax. Since you're new to benchmarking, you should definitely read my answer for caveats and gotchas about RDTSC. On arm, the TSC is a control register, accessible only in kernel mode. According to Microsoft: We strongly discourage using the RDTSC or RDTSCP processor instruction to directly query the TSC because you won't get reliable results on some versions of Windows, across live migrations of virtual machines, and on hardware systems Is there any way to read out the timestamp-counter on x86 CPUs in Python? I know that using rdtscp is bad and using rdtsc is even worse. I In ns_per_rdtsc_tick() to convert rdtsc to wall time __rdtsc() is used while next in benchmark tests __rdtscp(&aux) is used. rdstcp for the second measurement is useful, since it means the timestamp comes from after the load has executed. On Linux, read time(7) Using RDTSC instruction that returns CPU TSC (Time Stamp Counter) - rdtsc. It like lfence; rdtsc, not lfence;rdtsc;lfence which you sometimes actually want. Use lfence to order rdtsc, or use rdtscp. – In ns_per_rdtsc_tick() to convert rdtsc to wall time __rdtsc() is used while next in benchmark tests __rdtscp(&aux) is used. 3. As far as I know, the main difference in runtime ordering in a processor with respect to rdtsc and rdtscp instruction is that whether the execution waits until all previous instructions are Newer processors support a new instruction called RDTSCP that does exactly the same thing as RDTSC, except it is doing it in a serializing way (which means it waits for all Here's some good references to learn about the rdtsc and the rdtscp instructions: Stackoverflow thread; Intel's guide on benchmarking code with rdtsc(p) rdtscp reference; rdtsc reference If the CPU supports the RDTSCP instruction, so much the better. c. The PMU cycle counter runs at CPU frequency. The guest attempted to execute an RDTSCP instruction and the “enable RDTSCP” and “RDTSC exiting” VM-execution controls were both 1. If you run code that uses the intrinsic on hardware that doesn't support the rdtscp instruction, the results are unpredictable. Pour déterminer la prise en charge matérielle de cette instruction, appelez l’intrinsèque __cpuid avec InfoType=0x80000001 et vérifiez le bit 27 de CPUInfo[3] (EDX). The instructions just return, for rdtsc, the TimeStampCounter (TSC) and , for rdtscp, the TSC and MSR_TSC_AUX (an msr with a value to indicate which cpu you were running on when you did the rdtscp instruction). #[cfg(test)] use stdarch_test::assert_instr; /// Reads the current value of the processor’s time-stamp counter. For many years this was the best way to get high-precision timing information, but newer motherboards are now including dedicated timing devices which provide high-resolution timing information without the drawbacks of RDTSC. QueryPerformanceCounter should be an answer, but the returned frequency by QueryPerformanceFrequency is 10 MHz on Windows 10 and even less on Windows 7. 2,283; asked Feb 10, 2019 at 21:43. rdtsc is exactly correlated with wall-clock time (not counting system clock adjustments, so it's a perfect time source for steady_clock). Requirements. Assembly code to read the TSC. On the other hand, when reading posts about the asm code, I saw people saying that rdtsc instruction should be used in combination with cpuid or other fences to flush the pipeline, like this one. However, it isn't a serializing To do this you'd measure how long it takes to do nothing (just the RDTSC/RDTSCP instruction alone, while discarding dodgy measurements); then subtract the overhead of measuring from the "measuring something" results. Now, I want to use these time-stamp instructions across context rdtscp is much weaker than this, only draining the ROB but not the store buffer. The following code seems to work on Intel but not on ARM processors Thank you! The paper basically says RDTSCP is more precise than RDTSC. If you read some online docs on RDTSC, you'll see that it doesn't ensure instructions from after the RDTSC instruction aren't executed in the pipeline before the RDTSC instruction itself runs (nor that earlier instructions don't run afterwards). Modified 13 years, 11 months ago. RDTSCP is much faster on a (Skylake Xeon) Xeon Platinum 8160, returning 18 cycles about 40% of the time and 20 cycles about 60% of the time. After the previous function is created, create an exit handler for RDTSC : [EXIT_REASON_RDTSC] = handle_rdtsc. e. You don't have cpuid or lfence inside your measurement interval. I found the following info about TSC in wikipedia If I replace the RDTSC with RDTSCP I get much better results. You should read some intel manual. Not all of the clocks offered by clock_gettime will implement this fast method, RDTSC always writes its 64-bit result split into hi/lo halves in EDX and EAX, even in 64-bit mode (see the manual), unfortunately not packing the 64-bit TSC into just RAX. TSC feature bits in Linux¶ In summary, there is no way to guarantee the TSC remains in perfect You're supposed to look at an asm instruction set manual, like Intel's (links in the x86 tag wiki). You save a byte by just calling rdtsc, which is the way recommended in that awesome Intel On the other hand, rdtscp assembly instruction return a value indicating the CPU in which the TSC was read. 1 But it does not wait for previous stores to be globally visible, and subsequent instructions may begin execution before the read operation is performed. Works on both Windows and Linux. That's why extra work is needed after the asm statement. g. But trust me I really need that value, or at least some PoC that measures how long it takes the CPU to execute the CPUID instruction and reports if it suspects a VM. Why Use TSC? High Resolution: The TSC increments with each CPU cycle, providing fine-grained timing. The Visual C++ comparison code: __declspec(noinline) uint64_t __stdcall Rdtsc(void) { return On x86, it will [usually] invoke rdtsc [or a PET], and adjust the counter value to represent nanoseconds. I was wondering if there is a way to obtain this either with rdtscp or rdtsc. Thus, to use RDTSC effectively these cache effects must be taken into account. You should be measuring Reads the current value of the processor’s time-stamp counter (a 64-bit MSR) into the EDX:EAX registers and also reads the value of the IA32_TSC_AUX MSR (address C0000103H) into the What is the overhead of rdtsc instruction vs rdtscp instruction? What is the maximum usable resolution of rdtsc instruction vs rdtscp instruction across different cores? Does rdtsc Generates the rdtscp instruction, writes TSC_AUX[31:0] to memory, and returns the 64-bit Time Stamp Counter (TSC) result. Follow edited Nov 8, 2011 at 12:58. Still, I am worried about rdtsc accuracy across cores, since I've been reading some texts which implied that tsc is not synced between cores. See RDTSCP in NASM always returns the same value for more about how to measure throughput/latency/uops for a single instruction. This is typically Also, the rdtscp in your bench_start () is redundant due to the previous cpuid call. What would be the C equivalent of rlwinm (PPC Instruction) 3. However, patches have this line printk("[handle_rdtsc] fake rdtsc svm function is working\n"); but I cant find it in syslog. Follow answered Aug 5, 2014 at 22:33. Intel could either take the easy way out and simply increment the counter by the base I'm using RDTSC and not RDTSCP (same problem, assembly isn't my language) It is very slow Let's say that the equivalent Visual C++ code is called without inlining in 18-24 ticks of RDTSC, while the C# version, after the initial warmup, is called in 27-100 ticks of RDTSC. I recommend you simplify your rdtsc code substantially. It actually means that you cannot modify the value it returns if you dont make use of the TSC offset (which is broken anyways to spoof timings). __rdtsc. You signed out in another tab or window. I've noticed that while i can change rdtsc's result with ease, anytime i try enabling tsc scaling, tsc offsetting, or i try changing rdtscp's value directly, explorer. my patches for linux kernel to spoof rdtsc and make vm exit undetected - WCharacter/RDTSC-KVM-Handler . If the kernel to be measured is super tiny, rdtsc will give you less biased Intel has provided an RDTSCP instruction that’s more deterministic. Either patch does not work or somehow I failed to build them correctly. None Example Although an alternative method of prevention involves use of the RDTSCP instruction in place of RDTSC, we could not find any RDTSCP sandwiches in the samples in this study. (When in real-address mode rdtsc/rdtscp latency test; experiment to read TSC in one thread and pass results to another via memory - testing-laboratory/rdtscTest You want lfence;rdtsc to start the clock, and rdtscp;lfence to stop the clock, so the barriers are outside the timed interval. Here is a way is a way to use rdtscp() in C or C++ when using gcc. h> Remarks. However, rdtscp is only available on newer CPUs. The Processor ID is an MSR (Machine Specific Register) which therefore must be initialized by privileged system software, its purpose is to identify which "core" is executing the instruction. This makes implementation trickier. QueryPerformanceCounter and RDTSC. You normally wouldn't want it for you to wait for the store buffer to drain; you can wait for that with mfence. Similarly, subsequent instructions may begin execution before the read operation is performed. All gists Back to GitHub Sign in Sign up Sign in Sign up You signed in with another tab or window. I am not able to understand why this is If changing from RDTSCP to RDTSC does not change the overall performance by a detectable amount, then it is likely that neither of them is actually a problem, and the VTune analysis is biased. Microsoft offers two commonly used tools to facilitate this remote access, known as Remote Desktop Protocol (RDP) and Remote Desktop Connection (RDC). Then, the VDSO snippet will know how to access the values I am trying to profile a code for execution time on an x86-64 processor. h> 备注. Python wrapper around the RDTSCP instruction to get the cycle counter on X86 - Roguelazer/rdtsc. Timing, like everything, is more complicated than it first appears. The default cntvct_el0 based rte_rdtsc() provides a portable means to get a wall clock counter in user space. Improve this answer. Out of the generally recognized need, the rdtscp instruction was introduced. Using a CPUID after the second RDTSC probably isn't useful. 9. Collaborate outside of Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company So RDTSC is actually the wrong instruction, you need to use RDTSCP, and check that the CPU core used was the same across both measurements. 但是rdtsc是非序列化的，因此单独使用它不会阻止 CPU 对其进行重新排序。 所以我们可以使用这两个选项中的任何一个来防止重新排序： 2：这是对cpuidthen的调用rdtsc。cpuid是一个序列化调用。 The Win32 API has a so-called 'high performance counter' (QueryPerformanceCounter() and friends) but often it is neither precise enough nor reliable enough, due to high jitter. The exact definition of this counter has changed over time, but on recent CPUs it is a counter that increments at a fixed frequency with respect to wall clock time, so it is very useful as building block for a fast, accurate clock or measuring the time taken by Merge an old patch from 2011 that supports VM_EXIT catching, then adapt it to the modern kernel module, flesh it out, build it and you’re good with most things (this will bypass pafish if the implementation you created is good enough and doesn’t take too long to return a RDTSC call) Most of the code in this library is adapted from the implementation of a similar mechanism in DPDK project. If you need to measure short times (<<1ms) then always use RDTSC. A few late nights and a file full of #ifdefs later, I have the start of such a utility. RDTSC is the x86 instruction "ReaD TimeStamp Counter" - it reads a 64-bit counter that counts up at every clock cycle of your processor. To make matters worse, the guest is now a hypervisor itself, which means any applications run inside are in an L2 nested environment. Note that this is a different For solving the above problems, some more or less serializing instructions has to be used. – However, rdtscp is only available on newer CPUs. Intel’s documentation Access a wide range of self-help resources for Intel® product support. On my Xeon E5-2680 rdtsc counts reference cycles, not CPU core clock cycles. Updated 1 year, 2 months ago. You don't need to do 64-bit math carries your self, and you don't need to store the result of that operation as a double. Comment 37 Jason Vas Dias 2018-03-22 15:15:17 UTC The hypervisor should not be intercepting RDTSC/RDTSCP when Secure TSC is enabled. exe completely breaks down to the point where the entire computer freezes. rdtscp and lfence/rdtsc have the same exact upstream serialization properties On Intel processors. This is often caused by cache effects. Typically it runs at <= 100MHz. On AMD processors with a dispatch-serializing lfence, both sequences have also the same upstream serialization properties. Skip to main content. 6 Detect if processor has RDTSCP at compile time This is the code I am using in C to convert RDTSC clock ticks to time in usec. This course is an introductory guide to HyperDbg debugger, guiding you through the initial steps of usin rdtsc instruction. Leaving out the return and the call that may or may not prevent the CPU from seeing the second rdtsc in time for a reorder, it is unlikely (though possible!) that the CPU will reorder two rdtsc even if one is right after the other. They are very useful for timing relatively short pieces of code, especially First, rdtsc is not a serialising instruction, which means it can be executed out-of-order with respect to other instructions. No change to result values. This course is an introductory guide to HyperDbg debugger, guiding you through the initial steps of usin The rdtsc (Read Time-Stamp Counter) instruction is used to determine how many CPU ticks took place since the processor was reset. > > This is easily visible with cpu_get_tsc_freq(): with the current code > > I get random values as cpu_cc_freq; if I remplace the calls to > > rdmsr(MSR_TSC) with cpu_counter() I get the right frequency Elapsed TSC cycles (using RDTSC or RDTSCP) Instructions — using the RDPMC instruction with counter number (1<<30)+0; Core Cycles not halted — using the RDPMC instruction with counter number (1<<30)+1; Reference Cycles not halted — using the RDPMC instruction with counter number (1<<30)+2; If you have the ability to program the general GitHub is where people build software. the Intel Instruction Set Manual Vol 2A & B, as a more trusted source: The processor monotonically increments the time-stamp counter MSR every clock cycle and resets it to 0 whenever the processor is reset. It supplies the value in the TSC and a programmable 32-bit value. The RDTSC instruction returns the TSC in EDX: EAX. The TSC should be configured as the clock source for the Linux kernel at boot time. The CPU instruction pipeline may reorder non-dependent instructions in any Ideally, what I need is a form of rdtscp that is ordered with respect to the load whose latency is being measured, and not ordered explicitly with any other instruction. From the way the instruction works, we can also conclude that the initial value pointed to by __A doesn't matter: it's just a pointer to write-only storage for the result. Its opcode is 0F 31. Abonnez-Vous massivement dans votre chaîne DRC News TVAbonnez-vous | Partagez | Commentez | Aimez https://www. Copy HyperDbg >! tsc pid 490 code {90 90 90} condition {90 90 90} Or if you want to use assembly codes directly, you In linux, the gcc compiler has the intrinsic function __rdtsc to measure the cpu cycles. Intrinsic 体系结构; __rdtsc: x86、x64: 头文件<intrin. Find and fix vulnerabilities Codespaces. You I measured the overhead of clock_gettime: it is between 100 cycles and 10 000 cycles, while rdtsc directly is 20 cycles. So even the overhead The RDTSC instruction is not a serializing instruction. rdtscp can help for that, but other uses may need a full serializing instruction to make sure rdtsc instructions don't pass other insns, and that other insns don't pass it. Also it can be closely synchronized with the system clock, which makes it a good alternative of The RDTSC instruction is not a serializing instruction. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company __rdtscp() on some platforms is a compiler intrinsic for the RDTSCP instruction, which is the recently introduced serialized version of RDTSC ("Read Time Stamp Counter"), used for counting the number of processor cycles, for example in benchmarking or timer code. However, subsequent instructions may begin execution before the read operation is performed” – as stated in the Intel manual. GetSystemTimePreciseAsFileTime has 100 ns tick/step. The CPU instruction pipeline may reorder non-dependent instructions in any You mean the only difference between rdtsc and rdtscp is that rdtscp is serializing? I thought to get the TSC that's synchronized between cores you had to use rdtscp. This obviously doesnt mean that you cant use it to meassure the time difference between a few vm exits. Si vous exécutez du code qui utilise l’intrinsèque sur le matériel qui ne prend pas en charge l The cpuid instruction is a full barrier, preventing reordering in both directions, while rdtscp prevents reordering from above. Hello everybody! I can’t bypass the timing of function “rdtscp”. But, higher end arm arches allow this to be mapped for R/O access by userspace. This is the simplest way to handle 99% of VM_Exit checks that I use, however as this returns a static value, some software may check the actual timings, in which the example would fail. ppc EXTDRI instruction equivalent in C++. If software requires RDTSC to be executed only after all previous instructions have completed locally, it can either use RDTSCP I can't imagine how the second implementation can be justified due to the reason you have mentioned and stated in that Intel whitepaper. You may want to return a random number or emulate In order to prevent an inline rdtsc function from being moved across any loads/stores/other operations, you should both write the asm as __asm__ __volatile__ and include "memory" in the clobber list. On processors that support the Intel 64 architecture, the high-order 32 bits of each of RAX, RDX, and RCX are cleared. 5. Viewed 1k times 2 I programmed my own string matching algorithm, and I want to measure its time accuratly, to compare it with other algorithms to check if my implementation is better. I am trying to measure the latency of an operation by using rdtsc(). Currently, it supports the function monotonic_seconds which returns the seconds from some unspecified start point as a double precision floating point number. (On Skylake, Agner Fog measured it at 22 uops, with one per 32 cycle throughput. Without doing the latter, GCC is prevented from removing the asm or moving it across any instructions that could need the results (or change the inputs) of This project aims to stabilize and minimize the perceived time difference of 2 RDTSC calls and a vmexit (cpuid specifically) in programs running inside a KVM virtual machine. This section gives details on two different ways to handle cache So anything like rdtscp(&dummy) or _mm_lfence(); __rdtsc() to wait for all earlier instructions to complete will defeat that, and is mostly useful for microarchitectural experiments, not for a very short-duration piece of code you're optimizing. (Unless you're RDTSC vs. If software requires RDTSC to be executed only after all previous instructions have completed locally, it can either use RDTSCP Don't do that -using yourself directly the RDTSC machine instruction- (because your OS scheduler could reschedule other threads or processes at arbitrary moments, or slow down the clock). Assembling all this information, I attempted to write a cross-platform utility for fine grained timing. Although the TSC register seems like an ideal time stamp mechanism, here are circumstances in which it can't function reliably 56. ). RDTSC can be useful for timing a whole loop or longer sequence of instructions, larger than the OoO execution window of your CPU. 1. Generates the rdtsc instruction, which returns the processor time stamp. But it will have to be a custom bit, because I don't think GCC "knows about" that feature bit. e31d0e51-c9bb-42ca-bbe9-a81ffe662387. I read "How to Benchmark Code Execution Times on Intel IA-32 and IA- optimization; nasm; x86-64; windows64; rdtsc; RTC222. It does not necessarily wait until all previous instructions have been executed before reading the counter. fyi/Dbg3301. The RDTSC/RDTSCP instructions can only execute on the local core and can't provide any direct information about any other core. Processor time - how many cycles? this is related to wall clock time by processor <p>The RDTSC instruction is not a serializing instruction. 此例程仅可用作内部函数。 在后代硬件中，对 TSC 值的解释与早期版本 It's not particularly useful to time a single instruction with RDTSC like that. Syntax unsigned __int64 __rdtsc(); Return value. We use rdtscp at the end rather than cpuid; rdtsc as cpuid is an expensive instruction with high variance, so we want it outside the benchmarked region. gpou uodap exyg fqzzk drv wgzt tigaed rdardx iwrrr vvdw