Re: [PATCH v6 06/16] KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS

["Xu, Like" <like.xu@xxxxxxxxx>]

On 2021/5/17 16:33, Peter Zijlstra wrote:
> On Tue, May 11, 2021 at 10:42:04AM +0800, Like Xu wrote:
> > diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> > index 2f89fd599842..c791765f4761 100644
> > --- a/arch/x86/events/intel/core.c
> > +++ b/arch/x86/events/intel/core.c
> > @@ -3898,31 +3898,49 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
> >   	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 = (x86_pmu.flags & PMU_FL_PEBS_ALL) ?
> > +		cpuc->pebs_enabled : (cpuc->pebs_enabled & PEBS_COUNTER_MASK);
> > -	if (x86_pmu.flags & PMU_FL_PEBS_ALL)
> > -		arr[0].guest &= ~cpuc->pebs_enabled;
> > -	else
> > -		arr[0].guest &= ~(cpuc->pebs_enabled & PEBS_COUNTER_MASK);
> > -	*nr = 1;
> Instead of endlessly mucking about with branches, do we want something
> like this instead?

Fine to me. How about the commit message for your below patch:

x86/perf/core: Add pebs_capable to store valid PEBS_COUNTER_MASK value

The value of pebs_counter_mask will be accessed frequently
for repeated use in the intel_guest_get_msrs(). So it can be
optimized instead of endlessly mucking about with branches.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

> ---
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 2521d03de5e0..bcfba11196c8 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -2819,10 +2819,7 @@ static int handle_pmi_common(struct pt_regs *regs, u64 status)
>   	 * counters from the GLOBAL_STATUS mask and we always process PEBS
>   	 * events via drain_pebs().
>   	 */
> -	if (x86_pmu.flags & PMU_FL_PEBS_ALL)
> -		status &= ~cpuc->pebs_enabled;
> -	else
> -		status &= ~(cpuc->pebs_enabled & PEBS_COUNTER_MASK);
> +	status &= ~(cpuc->pebs_enabled & x86_pmu.pebs_capable);
>   
>   	/*
>   	 * PEBS overflow sets bit 62 in the global status register
> @@ -3862,10 +3859,7 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr)
>   	arr[0].msr = MSR_CORE_PERF_GLOBAL_CTRL;
>   	arr[0].host = intel_ctrl & ~cpuc->intel_ctrl_guest_mask;
>   	arr[0].guest = intel_ctrl & ~cpuc->intel_ctrl_host_mask;
> -	if (x86_pmu.flags & PMU_FL_PEBS_ALL)
> -		arr[0].guest &= ~cpuc->pebs_enabled;
> -	else
> -		arr[0].guest &= ~(cpuc->pebs_enabled & PEBS_COUNTER_MASK);
> +	arr[0].guest &= ~(cpuc->pebs_enabled & x86_pmu.pebs_capable);
>   	*nr = 1;
>   
>   	if (x86_pmu.pebs && x86_pmu.pebs_no_isolation) {
> @@ -5546,6 +5540,7 @@ __init int intel_pmu_init(void)
>   	x86_pmu.events_mask_len		= eax.split.mask_length;
>   
>   	x86_pmu.max_pebs_events		= min_t(unsigned, MAX_PEBS_EVENTS, x86_pmu.num_counters);
> +	x86_pmu.pebs_capable		= PEBS_COUNTER_MASK;
>   
>   	/*
>   	 * Quirk: v2 perfmon does not report fixed-purpose events, so
> @@ -5730,6 +5725,7 @@ __init int intel_pmu_init(void)
>   		x86_pmu.pebs_aliases = NULL;
>   		x86_pmu.pebs_prec_dist = true;
>   		x86_pmu.lbr_pt_coexist = true;
> +		x86_pmu.pebs_capable = ~0ULL;
>   		x86_pmu.flags |= PMU_FL_HAS_RSP_1;
>   		x86_pmu.flags |= PMU_FL_PEBS_ALL;
>   		x86_pmu.get_event_constraints = glp_get_event_constraints;
> @@ -6080,6 +6076,7 @@ __init int intel_pmu_init(void)
>   		x86_pmu.pebs_aliases = NULL;
>   		x86_pmu.pebs_prec_dist = true;
>   		x86_pmu.pebs_block = true;
> +		x86_pmu.pebs_capable = ~0ULL;
>   		x86_pmu.flags |= PMU_FL_HAS_RSP_1;
>   		x86_pmu.flags |= PMU_FL_NO_HT_SHARING;
>   		x86_pmu.flags |= PMU_FL_PEBS_ALL;
> @@ -6123,6 +6120,7 @@ __init int intel_pmu_init(void)
>   		x86_pmu.pebs_aliases = NULL;
>   		x86_pmu.pebs_prec_dist = true;
>   		x86_pmu.pebs_block = true;
> +		x86_pmu.pebs_capable = ~0ULL;
>   		x86_pmu.flags |= PMU_FL_HAS_RSP_1;
>   		x86_pmu.flags |= PMU_FL_NO_HT_SHARING;
>   		x86_pmu.flags |= PMU_FL_PEBS_ALL;
> diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
> index 27fa85e7d4fd..6f3cf81ccb1b 100644
> --- a/arch/x86/events/perf_event.h
> +++ b/arch/x86/events/perf_event.h
> @@ -805,6 +805,7 @@ struct x86_pmu {
>   	void		(*pebs_aliases)(struct perf_event *event);
>   	unsigned long	large_pebs_flags;
>   	u64		rtm_abort_event;
> +	u64		pebs_capable;
>   
>   	/*
>   	 * Intel LBR