linux-kernel - Re: [PATCH v6 07/16] KVM: x86/pmu: Reprogram PEBS event to emulate guest PEBS counter

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Date:   Tue, 18 May 2021 21:28:52 +0800
From:   "Xu, Like" <like.xu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, weijiang.yang@...el.com,
        Kan Liang <kan.liang@...ux.intel.com>, ak@...ux.intel.com,
        wei.w.wang@...el.com, eranian@...gle.com, liuxiangdong5@...wei.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v6 07/16] KVM: x86/pmu: Reprogram PEBS event to emulate
 guest PEBS counter

On 2021/5/17 17:14, Peter Zijlstra wrote:
> On Tue, May 11, 2021 at 10:42:05AM +0800, Like Xu wrote:
>> @@ -99,6 +109,7 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
>>   				  bool exclude_kernel, bool intr,
>>   				  bool in_tx, bool in_tx_cp)
>>   {
>> +	struct kvm_pmu *pmu = vcpu_to_pmu(pmc->vcpu);
>>   	struct perf_event *event;
>>   	struct perf_event_attr attr = {
>>   		.type = type,
>> @@ -110,6 +121,7 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
>>   		.exclude_kernel = exclude_kernel,
>>   		.config = config,
>>   	};
>> +	bool pebs = test_bit(pmc->idx, (unsigned long *)&pmu->pebs_enable);
>>   
>>   	attr.sample_period = get_sample_period(pmc, pmc->counter);
>>   
>> @@ -124,9 +136,23 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
>>   		attr.sample_period = 0;
>>   		attr.config |= HSW_IN_TX_CHECKPOINTED;
>>   	}
>> +	if (pebs) {
>> +		/*
>> +		 * The non-zero precision level of guest event makes the ordinary
>> +		 * guest event becomes a guest PEBS event and triggers the host
>> +		 * PEBS PMI handler to determine whether the PEBS overflow PMI
>> +		 * comes from the host counters or the guest.
>> +		 *
>> +		 * For most PEBS hardware events, the difference in the software
>> +		 * precision levels of guest and host PEBS events will not affect
>> +		 * the accuracy of the PEBS profiling result, because the "event IP"
>> +		 * in the PEBS record is calibrated on the guest side.
>> +		 */
>> +		attr.precise_ip = 1;
>> +	}
>>   
>>   	event = perf_event_create_kernel_counter(&attr, -1, current,
>> -						 intr ? kvm_perf_overflow_intr :
>> +						 (intr || pebs) ? kvm_perf_overflow_intr :
>>   						 kvm_perf_overflow, pmc);
> How would pebs && !intr be possible?

I don't think it's possible.

> Also; wouldn't this be more legible
> when written like:
>
> 	perf_overflow_handler_t ovf = kvm_perf_overflow;
>
> 	...
>
> 	if (intr)
> 		ovf = kvm_perf_overflow_intr;
>
> 	...
>
> 	event = perf_event_create_kernel_counter(&attr, -1, current, ovf, pmc);
>

Please yell if you don't like this:

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 711294babb97..a607f5a1b9cd 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -122,6 +122,8 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, 
u32 type,
                 .config = config,
         };
         bool pebs = test_bit(pmc->idx, (unsigned long *)&pmu->pebs_enable);
+       perf_overflow_handler_t ovf = (intr || pebs) ?
+               kvm_perf_overflow_intr : kvm_perf_overflow;

         attr.sample_period = get_sample_period(pmc, pmc->counter);

@@ -151,9 +153,7 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, 
u32 type,
                 attr.precise_ip = 1;
         }

-       event = perf_event_create_kernel_counter(&attr, -1, current,
-                                                (intr || pebs) ? 
kvm_perf_overflow_intr :
-                                                kvm_perf_overflow, pmc);
+       event = perf_event_create_kernel_counter(&attr, -1, current, ovf, pmc);
         if (IS_ERR(event)) {
                 pr_debug_ratelimited("kvm_pmu: event creation failed %ld 
for pmc->idx = %d\n",
                             PTR_ERR(event), pmc->idx);