linux-kernel - Re: [PATCH RFC v1 07/20] KVM: nVMX: Support the extended instruction info field

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Message-ID: <CABgObfa-vqWCenVvvTAoB773AQ+9a1OOT9n5hjqT=zZBDQbb+Q@mail.gmail.com>
Date: Tue, 18 Nov 2025 11:33:39 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Chao Gao <chao.gao@...el.com>
Cc: "Chang S. Bae" <chang.seok.bae@...el.com>, kvm <kvm@...r.kernel.org>, 
	"Kernel Mailing List, Linux" <linux-kernel@...r.kernel.org>, Sean Christopherson <seanjc@...gle.com>, 
	Zhao Liu <zhao1.liu@...el.com>
Subject: Re: [PATCH RFC v1 07/20] KVM: nVMX: Support the extended instruction
 info field

On Tue, Nov 18, 2025 at 2:39 AM Chao Gao <chao.gao@...el.com> wrote:
>
> On Tue, Nov 18, 2025 at 12:29:19AM +0100, Paolo Bonzini wrote:
> >Il mer 12 nov 2025, 02:54 Chao Gao <chao.gao@...el.com> ha scritto:
> >>
> >> Shouldn't we check guest's capabilities rather than host's,
> >>
> >> i.e., guest_cpu_cap_has(X86_FEATURE_APX)?
> >
> >As the manual says, you're free to use the extended field if
> >available, and it's faster.
>
> The point is, from the guest's perspective, the field is available iff the vCPU
> supports APX. KVM (L0) doesn't need to virtualize VMCS12's EII field if the vCPU
> doesn't have APX.

Well, it would be faster to just do it. But you're right, checking the
guest CPUID is consistent with other code, for example

        if (nested_cpu_has_vid(vmcs12))
                vmcs12->guest_intr_status = vmcs_read16(GUEST_INTR_STATUS);

Paolo