linux-kernel - Re: [PATCH 9/9] KVM: x86: Restrict XSAVE in cpu

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Date:   Sun, 19 Nov 2023 19:36:41 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 9/9] KVM: x86: Restrict XSAVE in cpu_caps based on KVM
 capabilities

On Fri, 2023-11-10 at 15:55 -0800, Sean Christopherson wrote:
> Restrict XSAVE in guest cpu_caps so that XSAVES dependencies on XSAVE are
> automatically handled instead of manually checking for host and guest
> XSAVE support.  Aside from modifying XSAVE in cpu_caps, this should be a
> glorified nop as KVM doesn't query guest XSAVE support (which is also why
> it wasn't/isn't a bug to leave XSAVE set in guest CPUID).
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/svm/svm.c | 2 +-
>  arch/x86/kvm/vmx/vmx.c | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 9e3a9191dac1..6fe2d7bf4959 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4315,8 +4315,8 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  	 * XSS on VM-Enter/VM-Exit.  Failure to do so would effectively give
>  	 * the guest read/write access to the host's XSS.
>  	 */
> +	guest_cpu_cap_restrict(vcpu, X86_FEATURE_XSAVE);
>  	guest_cpu_cap_change(vcpu, X86_FEATURE_XSAVES,
> -			     boot_cpu_has(X86_FEATURE_XSAVE) &&
>  			     boot_cpu_has(X86_FEATURE_XSAVES) &&
>  			     guest_cpu_cap_has(vcpu, X86_FEATURE_XSAVE));
>  
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 815692dc0aff..7645945af5c5 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7752,8 +7752,8 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  	 * to the guest.  XSAVES depends on CR4.OSXSAVE, and CR4.OSXSAVE can be
>  	 * set if and only if XSAVE is supported.
>  	 */
> -	if (boot_cpu_has(X86_FEATURE_XSAVE) &&
> -	    guest_cpu_cap_has(vcpu, X86_FEATURE_XSAVE))
> +	guest_cpu_cap_restrict(vcpu, X86_FEATURE_XSAVE);
> +	if (guest_cpu_cap_has(vcpu, X86_FEATURE_XSAVE))
>  		guest_cpu_cap_restrict(vcpu, X86_FEATURE_XSAVES);
>  	else
>  		guest_cpu_cap_clear(vcpu, X86_FEATURE_XSAVES);

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky