linux-kernel - Re: [PATCH] perf/x86/intel: Save/restore cpuc->active_pebs_data

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Message-ID: <70c86b5e-a8c8-fb6a-d4cf-0580a527ee5a@gmail.com>
Date:   Fri, 19 May 2023 15:40:53 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] perf/x86/intel: Save/restore cpuc->active_pebs_data_cfg
 when using guest PEBS

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@...cent.com>
>>
>> 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@...ux.intel.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Signed-off-by: Like Xu <likexu@...cent.com>
>> ---
> 
> Reviewed-by: Kan Liang <kan.liang@...ux.intel.com>
> 
> 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,
>>   		};
>>   	}