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

From: Xiaoyao Li <xiaoyao.li@intel.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: x86: Restore update of required xstate size in guest's CPUID
Date: Sat, 9 May 2020 10:09:00 +0800	[thread overview]
Message-ID: <dd7bfdb9-dcad-8a4b-29bb-c48d4c98b515@intel.com> (raw)
In-Reply-To: <20200508233749.3417-1-sean.j.christopherson@intel.com>

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@intel.com>
> Fixes: d87277414b851 ("kvm: x86: Cleanup vcpu->arch.guest_xstate_size")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> 
> 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) ||
> 