linux-kernel - Re: [PATCH v2] KVM: x86/pmu: Disable all vPMU features support on Intel hybrid CPUs

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Message-ID: <Y9j1aA84e8WG+FCr@hirez.programming.kicks-ass.net>
Date:   Tue, 31 Jan 2023 12:03:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jianfeng Gao <jianfeng.gao@...el.com>
Subject: Re: [PATCH v2] KVM: x86/pmu: Disable all vPMU features support on
 Intel hybrid CPUs

On Tue, Jan 31, 2023 at 04:50:31PM +0800, Like Xu wrote:
> From: Like Xu <likexu@...cent.com>
> 
> Disable KVM support for virtualizing PMUs on hosts with hybrid PMUs until
> KVM gains a sane way to enumeration the hybrid vPMU to userspace and/or
> gains a mechanism to let userspace opt-in to the dangers of exposing a
> hybrid vPMU to KVM guests.
> 
> Virtualizing a hybrid PMU, or at least part of a hybrid PMU, is possible,
> but it requires userspace to pin vCPUs to pCPUs to prevent migrating a
> vCPU between a big core and a little core, requires the VMM to accurately
> enumerate the topology to the guest (if exposing a hybrid CPU to the
> guest), and also requires the VMM to accurately enumerate the vPMU
> capabilities to the guest.
> 
> The last point is especially problematic, as KVM doesn't control which
> pCPU it runs on when enumerating KVM's vPMU capabilities to userspace.
> For now, simply disable vPMU support on hybrid CPUs to avoid inducing
> seemingly random #GPs in guests.
> 
> Reported-by: Jianfeng Gao <jianfeng.gao@...el.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Like Xu <likexu@...cent.com>

This seems reasonable; Paolo, will you take this through the KVM tree?

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

> ---
> v1: https://lore.kernel.org/all/20230120004051.2043777-1-seanjc@google.com/
>  arch/x86/kvm/pmu.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index 79988dafb15b..6a3995657e1e 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -166,9 +166,11 @@ static inline void kvm_init_pmu_capability(const struct kvm_pmu_ops *pmu_ops)
>  
>  	 /*
>  	  * For Intel, only support guest architectural pmu
> -	  * on a host with architectural pmu.
> +	  * on a non-hybrid host with architectural pmu.
>  	  */
> -	if ((is_intel && !kvm_pmu_cap.version) || !kvm_pmu_cap.num_counters_gp)
> +	if (!kvm_pmu_cap.num_counters_gp ||
> +	    (is_intel && (!kvm_pmu_cap.version ||
> +			  boot_cpu_has(X86_FEATURE_HYBRID_CPU))))
>  		enable_pmu = false;
>  
>  	if (!enable_pmu) {
> -- 
> 2.39.1
>