Re: [PATCH 4/7] KVM: x86/pmu: Not to generate PEBS records for emulated instructions

[Sean Christopherson <seanjc@google.com>]

"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

> 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.

> +			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
>