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

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Date:   Wed, 6 Oct 2021 23:04:04 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.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()

On Tue, Sep 21, 2021, Vitaly Kuznetsov wrote:
> 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 ...

Yep, I would also have preferred that KVM force userspace to avoid ioctls() that
cannot work, but that ship has sailed.