Re: [PATCH] perf/x86/intel: Save/restore cpuc->active_pebs_data_cfg when using guest PEBS

[Like Xu]

On 19/5/2023 12:31 am, Liang, Kan wrote:
> On 2023-05-17 9:38 a.m., Like Xu wrote:
> > From: Like Xu <likexu@xxxxxxxxxxx>
> >
> > After commit b752ea0c28e3 ("perf/x86/intel/ds: Flush PEBS DS when changing
> > PEBS_DATA_CFG"), the cpuc->pebs_data_cfg may save some bits that are not
> > supported by real hardware, such as PEBS_UPDATE_DS_SW. This would cause
> > the VMX hardware MSR switching mechanism to save/restore invalid values
> > for PEBS_DATA_CFG MSR, thus crashing the host when PEBS is used for guest.
>
> I believe we clear the SW bit when it takes effect.
>
> +	if (cpuc->pebs_data_cfg & PEBS_UPDATE_DS_SW) {
> +		cpuc->pebs_data_cfg = pebs_data_cfg;
> +		pebs_update_threshold(cpuc);
> +	}
>
> I think the SW bit can only be seen in a shot period between add() and
> enable(). Is it caused by a VM enter which just happens on the period?

What happens here is that when *intel_pmu_pebs_del()* is called,
the pebs_update_state() also triggers:
	cpuc->pebs_data_cfg |= PEBS_UPDATE_DS_SW;
and the new value will then be used for the next kvm_entry.

The KVM created pebs perf_event is not added/enabled at this point
and the cpuc->pebs_data_cfg strangely holds a non-zero value.

Perhaps there is more room for perf fixes here, but for guest pebs usages,
using active_pebs_data_cfg in intel_guest_get_msrs() is part of what is needed.

> > Fix it by using the active host value from cpuc->active_pebs_data_cfg.
>
> I don't see a problem of using active_pebs_data_cfg, since it reflects
> the current MSR setting. Just curious about how it's triggered.
>
> > Cc: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> > ---
>
> Reviewed-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
>
> Thanks,
> Kan
>
> >   arch/x86/events/intel/core.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> > index 070cc4ef2672..89b9c1cebb61 100644
> > --- a/arch/x86/events/intel/core.c
> > +++ b/arch/x86/events/intel/core.c
> > @@ -4074,7 +4074,7 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
> >   	if (x86_pmu.intel_cap.pebs_baseline) {
> >   		arr[(*nr)++] = (struct perf_guest_switch_msr){
> >   			.msr = MSR_PEBS_DATA_CFG,
> > -			.host = cpuc->pebs_data_cfg,
> > +			.host = cpuc->active_pebs_data_cfg,
> >   			.guest = kvm_pmu->pebs_data_cfg,
> >   		};
> >   	}