Re: [PATCH v4 10/10] perf/x86/intel: Check only PMC bits in PEBS_ENABLED when detecting host PEBS usage

[Mi, Dapeng]

Reviewed-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>

Thanks.

On 9/22/2026 3:14 AM, Sean Christopherson wrote:
> When checking to see if guest PEBS usage should be disabled due to host
> PEBS usage, filter out non-counter bits in PEBS_ENABLE to avoid getting
> false positives in the unlikely scenario that "Enable Precise Store" is
> enabled without PEBS actually being active for any PMCs.
>
> Note, the other "hidden" usage of pebs_mask is already explicitly masked
> with intel_ctrl:
>
>   .guest = intel_ctrl & ~cpuc->intel_ctrl_exclude_guest_mask & ~pebs_mask,
>
> I.e. removing the non-counter bits from pebs_mask doesn't change the value
> that is loaded into arr[global_ctrl].guest.
>
> Fixes: 854250329c02 ("KVM: x86/pmu: Disable guest PEBS temporarily in two rare situations")
> Suggested-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> Reported-by: Sashiko Bot <sashiko-bot@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/20260508234018.F06AEC2BCB0@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/events/intel/core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index d9315bb2f221..1ff0586c7b71 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -5323,7 +5323,7 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr,
>  	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>  	struct perf_guest_switch_msr *arr = cpuc->guest_switch_msrs;
>  	u64 intel_ctrl = hybrid(cpuc->pmu, intel_ctrl);
> -	u64 pebs_mask = cpuc->pebs_enabled & x86_pmu.pebs_capable;
> +	u64 pebs_mask = intel_ctrl & cpuc->pebs_enabled & x86_pmu.pebs_capable;
>  	u64 guest_pebs_mask;
>  	int global_ctrl;
>  
> @@ -5375,7 +5375,7 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr,
>  	 * the guest wants to use for PEBS, (c) are not excluded from counting
>  	 * in the guest, and (d) _are_ excluded from counting in the host.
>  	 */
> -	guest_pebs_mask = pebs_mask & intel_ctrl & guest_pebs->enable &
> +	guest_pebs_mask = pebs_mask & guest_pebs->enable &
>  			  ~cpuc->intel_ctrl_exclude_guest_mask &
>  			  cpuc->intel_ctrl_exclude_host_mask;
>