Re: [PATCH v2 43/49] KVM: x86: Update OS{XSAVE,PKE} bits in guest CPUID irrespective of host support

[Maxim Levitsky]

On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
> When making runtime CPUID updates, change OSXSAVE and OSPKE even if their
> respective base features (XSAVE, PKU) are not supported by the host.  KVM
> already incorporates host support in the vCPU's effective reserved CR4 bits.
> I.e. OSXSAVE and OSPKE can be set if and only if the host supports them.
> 
> And conversely, since KVM's ABI is that KVM owns the dynamic OS feature
> flags, clearing them when they obviously aren't supported and thus can't
> be enabled is arguably a fix.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 8256fc657c6b..552e65ba5efa 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -336,10 +336,8 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
>  
>  	best = kvm_find_cpuid_entry(vcpu, 1);
>  	if (best) {
> -		/* Update OSXSAVE bit */
> -		if (boot_cpu_has(X86_FEATURE_XSAVE))
> -			cpuid_entry_change(best, X86_FEATURE_OSXSAVE,
> -					   kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE));
> +		cpuid_entry_change(best, X86_FEATURE_OSXSAVE,
> +				   kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE));
>  
>  		cpuid_entry_change(best, X86_FEATURE_APIC,
>  			   vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);
> @@ -351,7 +349,7 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
>  	}
>  
>  	best = kvm_find_cpuid_entry_index(vcpu, 7, 0);
> -	if (best && boot_cpu_has(X86_FEATURE_PKU))
> +	if (best)
>  		cpuid_entry_change(best, X86_FEATURE_OSPKE,
>  				   kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE));
>  

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky