"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@xxxxxxxxxxx>
>
> The KVM accumulate an enabeld counter for at least INSTRUCTIONS or
Probably just "KVM" instead of "the KVM"?
s/enabeld/enabled
> 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@xxxxxxxxxxx>
> ---
> 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.
> + 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
>
