Re: [PATCH] KVM: x86: Restore update of required xstate size in guest's CPUID

[Xiaoyao Li]

On 5/9/2020 7:37 AM, Sean Christopherson wrote:
> Restore a guest CPUID update that was unintentional collateral damage
> when the per-vCPU guest_xstate_size field was removed.

It's really unintentional. None of us noticed it. :(

It's good that you catch it!

> Cc: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> Fixes: d87277414b851 ("kvm: x86: Cleanup vcpu->arch.guest_xstate_size")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>
> There's nothing more thrilling than watching bisect home in on your own
> commits, only to land on someone else's on the very last step.
>
>   arch/x86/kvm/cpuid.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 35845704cf57a..cd708b0b460a0 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -84,11 +84,13 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
>   				   kvm_read_cr4_bits(vcpu, X86_CR4_PKE));
>   
>   	best = kvm_find_cpuid_entry(vcpu, 0xD, 0);
> -	if (!best)
> +	if (!best) {
>   		vcpu->arch.guest_supported_xcr0 = 0;
> -	else
> +	} else {
>   		vcpu->arch.guest_supported_xcr0 =
>   			(best->eax | ((u64)best->edx << 32)) & supported_xcr0;
> +		best->ebx = xstate_required_size(vcpu->arch.xcr0, false);
> +	}
>   
>   	best = kvm_find_cpuid_entry(vcpu, 0xD, 1);
>   	if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||