linux-kernel - Re: [PATCH v2 2/2] KVM: x86/pmu: Add PEBS support for SPR and future non-hybird models

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Date:   Tue, 11 Oct 2022 19:09:21 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jim Mattson <jmattson@...gle.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 2/2] KVM: x86/pmu: Add PEBS support for SPR and future
 non-hybird models

Kind of a nit, but I would prefer a shortlog that talks about the pdit/pdir
stuff and not a "enable PEBS" bucket.

On Thu, Sep 22, 2022, Like Xu wrote:
> @@ -140,6 +149,16 @@ static void kvm_perf_overflow(struct perf_event *perf_event,
>  	__kvm_perf_overflow(pmc, true);
>  }
>  
> +static bool need_max_precise(struct kvm_pmc *pmc)
> +{
> +	if (pmc->idx == 0 && x86_match_cpu(vmx_pebs_pdist_cpu))
> +		return true;
> +	if (pmc->idx == 32 && x86_match_cpu(vmx_pebs_pdir_cpu))
> +		return true;
> +
> +	return false;
> +}
> +
>  static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
>  				  u64 config, bool exclude_user,
>  				  bool exclude_kernel, bool intr)
> @@ -181,11 +200,11 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
>  		 * the accuracy of the PEBS profiling result, because the "event IP"
>  		 * in the PEBS record is calibrated on the guest side.
>  		 *
> -		 * On Icelake everything is fine. Other hardware (GLC+, TNT+) that
> +		 * On Icelake everything is fine. Other hardware (TNT+) that
>  		 * could possibly care here is unsupported and needs changes.

This part of the comment is still somewhat stale, and for me at least it's not at
all helpful.  

>  		 */
>  		attr.precise_ip = 1;
> -		if (x86_match_cpu(vmx_icl_pebs_cpu) && pmc->idx == 32)
> +		if (need_max_precise(pmc))
>  			attr.precise_ip = 3;

What about writing this as:

		attr.precise_ip = pmc_get_pebs_precision(pmc);

(or whatever name is appropriate for "pebs_precision").

Then in the helper, add comments to explaint the magic numbers and the interaction
with PDIST and PDIR.  Bonus points if #defines for the the magic numbers can be
added somewher

				 *  0 - SAMPLE_IP can have arbitrary skid
				 *  1 - SAMPLE_IP must have constant skid
				 *  2 - SAMPLE_IP requested to have 0 skid
				 *  3 - SAMPLE_IP must have 0 skid

static u64 pmc_get_pebs_precision(struct kvm_pmc *pmc)
{
	/* Comment that explains why PDIST/PDIR require 0 skid? */
	if ((pmc->idx == 0 && x86_match_cpu(vmx_pebs_pdist_cpu)) ||
	    (pmc->idx == 32 && x86_match_cpu(vmx_pebs_pdir_cpu)))
		return 3;

	/* Comment about constant skid? */
	return 1;
}


>  	}
>  
> diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
> index c5e5dfef69c7..4dc4bbe18821 100644
> --- a/arch/x86/kvm/vmx/capabilities.h
> +++ b/arch/x86/kvm/vmx/capabilities.h
> @@ -398,7 +398,9 @@ static inline bool vmx_pt_mode_is_host_guest(void)
>  
>  static inline bool vmx_pebs_supported(void)
>  {
> -	return boot_cpu_has(X86_FEATURE_PEBS) && kvm_pmu_cap.pebs_ept;
> +	return boot_cpu_has(X86_FEATURE_PEBS) &&
> +		!boot_cpu_has(X86_FEATURE_HYBRID_CPU) &&

This belongs in a separate patch, and it should be ordered before patch 1 so that
there's no window where KVM can see pebs_ept==1 on a hybrid CPU.

Actually, shouldn't everything in this patch land before core enabling?

> +		kvm_pmu_cap.pebs_ept;

Please align indentation:

	return boot_cpu_has(X86_FEATURE_PEBS) &&
	       !boot_cpu_has(X86_FEATURE_HYBRID_CPU) &&
	       kvm_pmu_cap.pebs_ept;