linux-kernel - Re: [PATCH v3 1/3] KVM: x86/pmu: Disable guest PEBS on hybird cpu due to heterogeneity

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Date:   Fri, 20 Jan 2023 00:47:27 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v3 1/3] KVM: x86/pmu: Disable guest PEBS on hybird cpu
 due to heterogeneity

On Wed, Nov 09, 2022, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> From vPMU enabling perspective, KVM does not have proper support for
> hybird x86 core. The reported perf_capabilities value (e.g. the format
> of pebs record) depends on the type of cpu the kvm-intel module is init.
> When a vcpu of one pebs format migrates to a vcpu of another pebs format,
> the incorrect parsing of pebs records by guest can make profiling data
> analysis extremely problematic.
> 
> The safe way to fix this is to disable this part of the support until the
> guest recognizes that it is running on the hybird cpu, which is appropriate
> at the moment given that x86 hybrid architectures are not heavily touted
> in the data center market.
> 
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
>  arch/x86/kvm/vmx/capabilities.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
> index cd2ac9536c99..ea0498684048 100644
> --- a/arch/x86/kvm/vmx/capabilities.h
> +++ b/arch/x86/kvm/vmx/capabilities.h
> @@ -392,7 +392,9 @@ 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) &&
> +	       kvm_pmu_cap.pebs_ept;

I assume the patch I just posted[*] to disable the vPMU entirely is sufficient, or
do we need this as well in order to hide X86_FEATURE_DS and X86_FEATURE_DTES64?

[*] https://lore.kernel.org/all/20230120004051.2043777-1-seanjc@google.com