linux-kernel - Re: [PATCH V7 15/18] KVM: x86/pmu: Disable guest PEBS temporarily in two rare situations

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Date:   Fri, 2 Jul 2021 14:46:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Zhu Lingshan <lingshan.zhu@...el.com>
Cc:     pbonzini@...hat.com, bp@...en8.de, seanjc@...gle.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, weijiang.yang@...el.com,
        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.linux@...il.com, Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH V7 15/18] KVM: x86/pmu: Disable guest PEBS temporarily in
 two rare situations

On Tue, Jun 22, 2021 at 05:43:03PM +0800, Zhu Lingshan wrote:
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 22386c1a32b4..8bf494f8af3e 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -3970,8 +3970,15 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
>  		.guest = pebs_mask & ~cpuc->intel_ctrl_host_mask,
>  	};
>  
> -	/* Set hw GLOBAL_CTRL bits for PEBS counter when it runs for guest */
> -	arr[0].guest |= arr[*nr].guest;
> +	if (arr[*nr].host) {
> +		/* Disable guest PEBS if host PEBS is enabled. */
> +		arr[*nr].guest = 0;
> +	} else {
> +		/* Disable guest PEBS for cross-mapped PEBS counters. */
> +		arr[*nr].guest &= ~pmu->host_cross_mapped_mask;
> +		/* Set hw GLOBAL_CTRL bits for PEBS counter when it runs for guest */
> +		arr[0].guest |= arr[*nr].guest;
> +	}

Not saying I disagree, but is there any way for the guest to figure out
why things aren't working? Is there like a guest log we can dump
something in?

> +void intel_pmu_cross_mapped_check(struct kvm_pmu *pmu)
> +{
> +	struct kvm_pmc *pmc = NULL;
> +	int bit;
> +
> +	for_each_set_bit(bit, (unsigned long *)&pmu->global_ctrl,
> +			 X86_PMC_IDX_MAX) {
> +		pmc = kvm_x86_ops.pmu_ops->pmc_idx_to_pmc(pmu, bit);
> +
> +		if (!pmc || !pmc_speculative_in_use(pmc) ||
> +		    !pmc_is_enabled(pmc))
> +			continue;
> +
> +		if (pmc->perf_event && (pmc->idx != pmc->perf_event->hw.idx))
> +			pmu->host_cross_mapped_mask |=
> +				BIT_ULL(pmc->perf_event->hw.idx);

{ } again.

> +	}
> +}