On 10/26/2023 8:19 PM, Jim Mattson wrote:
On Wed, Oct 25, 2023 at 7:14 PM Mi, Dapeng <dapeng1.mi@xxxxxxxxxxxxxxx> wrote:On 10/25/2023 8:35 PM, Jim Mattson wrote:On Wed, Oct 25, 2023 at 4:23 AM Mi, Dapeng <dapeng1.mi@xxxxxxxxxxxxxxx> wrote:On 10/24/2023 9:03 PM, Jim Mattson wrote:On Tue, Oct 24, 2023 at 12:51 AM Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx> wrote:Along with the CPU HW's upgrade and optimization, the count of LLC misses event for running loop() helper could be 0 just like seen on Sapphire Rapids. So modify the lower limit of possible count range for LLC misses events to 0 to avoid LLC misses event test failure on Sapphire Rapids.I'm not convinced that these tests are really indicative of whether or not the PMU is working properly. If 0 is allowed for llc misses, for instance, doesn't this sub-test pass even when the PMU is disabled? Surely, we can do better.Considering the testing workload is just a simple adding loop, it's reasonable and possible that it gets a 0 result for LLC misses and branch misses events. Yeah, I agree the 0 count makes the results not so credible. If we want to avoid these 0 count values, we may have to complicate the workload, such as adding flush cache instructions, or something like that (I'm not sure if there are instructions which can force branch misses). How's your idea about this?CLFLUSH is probably a good way to ensure cache misses. IBPB may be a good way to ensure branch mispredictions, or IBRS on parts without eIBRS.Thanks Jim for the information. I'm not familiar with IBPB/IBRS instructions, but just a glance, it looks there two instructions are some kind of advanced instructions, Not all Intel CPUs support these instructions and not sure if AMD has similar instructions. It would be better if there are more generic instruction to trigger branch miss. Anyway I would look at the details and come back again.IBPB and IBRS are not instructions. IBPB (indirect branch predictor barrier) is triggered by setting bit 0 of the IA32_PRED_CMD MSR. IBRS (indirect branch restricted speculation) is triggered by setting bit 0 of the IA32_SPEC_CTRL MSR. It is true that the desired behavior of IBRS (causing branch mispredictions) is only exhibited by certain older parts. However, IBPB is now universally available, as it is necessary to mitigate many speculative execution attacks. For Intel documentation, see https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/cpuid-enumeration-and-architectural-msrs.html. If you don't want to use these, you could train a branch to go one way prior to measurement, and then arrange for the branch under test go the other way.
Thanks Jim. From my point of view, IBPB is still some kind of extended feature which may be not supported on some older platforms. Considering kvm-unit-tests could still be run on these old platforms, IBPB seems not the best choice. I'm thinking an alternative way is to use the 'rdrand' instruction to get a random value, and then call jmp instruction base on the random value results. That would definitely cause branch misses. This looks more generic.
Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx> --- x86/pmu.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/x86/pmu.c b/x86/pmu.c index 0def28695c70..7443fdab5c8a 100644 --- a/x86/pmu.c +++ b/x86/pmu.c @@ -35,7 +35,7 @@ struct pmu_event { {"instructions", 0x00c0, 10*N, 10.2*N}, {"ref cycles", 0x013c, 1*N, 30*N}, {"llc references", 0x4f2e, 1, 2*N}, - {"llc misses", 0x412e, 1, 1*N}, + {"llc misses", 0x412e, 0, 1*N}, {"branches", 0x00c4, 1*N, 1.1*N}, {"branch misses", 0x00c5, 0, 0.1*N}, }, amd_gp_events[] = { -- 2.34.1
