Re: [PATCH] KVM: x86/pmu: Disallow legacy LBRs if architectural LBRs are available

[Like Xu <like.xu.linux@gmail.com>]

On 28/1/2023 8:14 am, Sean Christopherson wrote:
> Disallow enabling LBR support if the CPU supports architectural LBRs.
> Traditional LBR support is absent on CPU models that have architectural
> LBRs, and KVM doesn't yet support arch LBRs, i.e. KVM will pass through
> non-existent MSRs if userspace enables LBRs for the guest.

True, we have call_trace due to MSR_ARCH_LBR_FROM_0 (0x1500) for example.

> 
> Cc: stable@vger.kernel.org
> Cc: Yang Weijiang <weijiang.yang@intel.com>
> Cc: Like Xu <like.xu.linux@gmail.com>

Tested-by: Like Xu <likexu@tencent.com>

> Reported-by: Paolo Bonzini <pbonzini@redhat.com>

Fixes: 145dfad998ea ("KVM: VMX: Advertise PMU LBRs if and only if perf supports 
LBRs")

> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> 
> Am I missing something that would prevent this scenario?
> 
>   arch/x86/kvm/vmx/vmx.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 8f0f67c75f35..77ee6b4a5ec4 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7761,9 +7761,11 @@ static u64 vmx_get_perf_capabilities(void)
>   	if (boot_cpu_has(X86_FEATURE_PDCM))
>   		rdmsrl(MSR_IA32_PERF_CAPABILITIES, host_perf_cap);
>   
> -	x86_perf_get_lbr(&lbr);
> -	if (lbr.nr)
> -		perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;
> +	if (!cpu_feature_enabled(X86_FEATURE_ARCH_LBR)) {

To avoid changing this again in the Arch lbr enabling part, how about:

	x86_perf_get_lbr(&lbr);
	if (lbr.nr && cpu_feature_enabled(X86_FEATURE_ARCH_LBR) ==
	    kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR))
		perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;

?

> +		x86_perf_get_lbr(&lbr);
> +		if (lbr.nr)
> +			perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;
> +	}
>   
>   	if (vmx_pebs_supported()) {
>   		perf_cap |= host_perf_cap & PERF_CAP_PEBS_MASK;
> 
> base-commit: 2de154f541fc5b9f2aed3fe06e218130718ce320