Messages in this thread | | | Date | Thu, 21 Jul 2022 10:22:14 +0800 | Subject | Re: [PATCH 4/7] KVM: x86/pmu: Not to generate PEBS records for emulated instructions | From | Like Xu <> |
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On 21/7/2022 8:51 am, Sean Christopherson wrote: > "Don't" instead of "Not to". Not is an adverb, not a verb itself. > > On Wed, Jul 13, 2022, Like Xu wrote: >> From: Like Xu <likexu@tencent.com> >> >> The KVM accumulate an enabeld counter for at least INSTRUCTIONS or > > Probably just "KVM" instead of "the KVM"? > > s/enabeld/enabled
Applied, thanks.
> >> BRANCH_INSTRUCTION hw event from any KVM emulated instructions, >> generating emulated overflow interrupt on counter overflow, which >> in theory should also happen when the PEBS counter overflows but >> it currently lacks this part of the underlying support (e.g. through >> software injection of records in the irq context or a lazy approach). >> >> In this case, KVM skips the injection of this BUFFER_OVF PMI (effectively >> dropping one PEBS record) and let the overflow counter move on. The loss >> of a single sample does not introduce a loss of accuracy, but is easily >> noticeable for certain specific instructions. >> >> This issue is expected to be addressed along with the issue >> of PEBS cross-mapped counters with a slow-path proposal. >> >> Fixes: 79f3e3b58386 ("KVM: x86/pmu: Reprogram PEBS event to emulate guest PEBS counter") >> Signed-off-by: Like Xu <likexu@tencent.com> >> --- >> arch/x86/kvm/pmu.c | 11 ++++++++--- >> 1 file changed, 8 insertions(+), 3 deletions(-) >> >> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c >> index 02f9e4f245bd..08ee0fed63d5 100644 >> --- a/arch/x86/kvm/pmu.c >> +++ b/arch/x86/kvm/pmu.c >> @@ -106,9 +106,14 @@ static inline void __kvm_perf_overflow(struct kvm_pmc *pmc, bool in_pmi) >> return; >> >> if (pmc->perf_event && pmc->perf_event->attr.precise_ip) { >> - /* Indicate PEBS overflow PMI to guest. */ >> - skip_pmi = __test_and_set_bit(GLOBAL_STATUS_BUFFER_OVF_BIT, >> - (unsigned long *)&pmu->global_status); >> + if (!in_pmi) { >> + /* The emulated instructions does not generate PEBS records. */ > > This needs a better comment. IIUC, it's not that they don't generate records, > it's that KVM is _choosing_ to not generate records to hack around a different > bug(s). If that's true a TODO or FIXME would also be nice.
Indeed, to understand more of the context, this part will look like this:
if (!in_pmi) { /* * TODO: KVM is currently _choosing_ to not generate records * for emulated instructions, avoiding BUFFER_OVF PMI when * there are no records. Strictly speaking, it should be done * as well in the right context to improve sampling accuracy. */ skip_pmi = true; } else { /* Indicate PEBS overflow PMI to guest. */ skip_pmi = __test_and_set_bit(GLOBAL_STATUS_BUFFER_OVF_BIT, (unsigned long *)&pmu->global_status); }
, what do you think ?
> >> + skip_pmi = true; >> + } else { >> + /* Indicate PEBS overflow PMI to guest. */ >> + skip_pmi = __test_and_set_bit(GLOBAL_STATUS_BUFFER_OVF_BIT, >> + (unsigned long *)&pmu->global_status); >> + } >> } else { >> __set_bit(pmc->idx, (unsigned long *)&pmu->global_status); >> } >> -- >> 2.37.0 >>
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