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Date:   Mon, 17 May 2021 11:14:36 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Like Xu <like.xu@linux.intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, weijiang.yang@intel.com,
        Kan Liang <kan.liang@linux.intel.com>, ak@linux.intel.com,
        wei.w.wang@intel.com, eranian@google.com, liuxiangdong5@huawei.com,
        linux-kernel@vger.kernel.org, x86@kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v6 07/16] KVM: x86/pmu: Reprogram PEBS event to emulate
 guest PEBS counter
Message-ID: <YKIz/J1HoOvbmR42@hirez.programming.kicks-ass.net>
References: <20210511024214.280733-1-like.xu@linux.intel.com>
 <20210511024214.280733-8-like.xu@linux.intel.com>
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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? 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);