linux-kernel - Re: [PATCH v2 06/10] KVM: x86: Fold fx_init() into kvm_arch_vcpu

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Date:   Tue, 21 Sep 2021 16:52:16 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Reiji Watanabe <reijiw@...gle.com>
Subject: Re: [PATCH v2 06/10] KVM: x86: Fold fx_init() into
 kvm_arch_vcpu_create()

Sean Christopherson <seanjc@...gle.com> writes:

> Move the few bits of relevant fx_init() code into kvm_arch_vcpu_create(),
> dropping the superfluous check on vcpu->arch.guest_fpu that was blindly
> and wrongly added by commit ed02b213098a ("KVM: SVM: Guest FPU state
> save/restore not needed for SEV-ES guest").

I have more questions to the above mentioned commit: why is it OK to
'return 0' from kvm_vcpu_ioctl_x86_set_xsave() without writing anything
to 'guest_xsave'? Same goes to kvm_arch_vcpu_ioctl_get_fpu(). Whould't
it be better to throw an error as we can't actually get this information
for encrypted guests? It's probably too late to change this now I
suppose ...

>
> Note, KVM currently allocates and then frees FPU state for SEV-ES guests,
> rather than avoid the allocation in the first place.  While that approach
> is inarguably inefficient and unnecessary, it's a cleanup for the future.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 16 ++++------------
>  1 file changed, 4 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6fd3fe21863e..ec61b90d9b73 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10614,17 +10614,6 @@ static int sync_regs(struct kvm_vcpu *vcpu)
>  	return 0;
>  }
>  
> -static void fx_init(struct kvm_vcpu *vcpu)
> -{
> -	if (!vcpu->arch.guest_fpu)
> -		return;
> -
> -	fpstate_init(&vcpu->arch.guest_fpu->state);
> -	if (boot_cpu_has(X86_FEATURE_XSAVES))
> -		vcpu->arch.guest_fpu->state.xsave.header.xcomp_bv =
> -			host_xcr0 | XSTATE_COMPACTION_ENABLED;
> -}
> -
>  void kvm_free_guest_fpu(struct kvm_vcpu *vcpu)
>  {
>  	if (vcpu->arch.guest_fpu) {
> @@ -10703,7 +10692,10 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
>  		pr_err("kvm: failed to allocate vcpu's fpu\n");
>  		goto free_user_fpu;
>  	}
> -	fx_init(vcpu);
> +	fpstate_init(&vcpu->arch.guest_fpu->state);
> +	if (boot_cpu_has(X86_FEATURE_XSAVES))
> +		vcpu->arch.guest_fpu->state.xsave.header.xcomp_bv =
> +			host_xcr0 | XSTATE_COMPACTION_ENABLED;
>  
>  	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
>  	vcpu->arch.reserved_gpa_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu);

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly