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

[Sean Christopherson]

On Tue, Jan 31, 2023, Like Xu wrote:
> From: Like Xu <likexu@xxxxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> ---
> v1: https://lore.kernel.org/all/20230120004051.2043777-1-seanjc@xxxxxxxxxx/
>  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))))

Why do this here instead of in perf_get_x86_pmu_capability()[*]?  The issue isn't
restricted to Intel CPUs, it just so happens that Intel is the only x86 vendor
that has shipped hybrid CPUs/PMUs.  Similarly, it's entirely possible to create a
hybrid CPU with a fully homogeneous PMU.  IMO KVM should rely on the PMU's is_hybrid()
and not the generic X86_FEATURE_HYBRID_CPU flag.

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