linux-kernel - Re: [PATCH] kvm: x86: Report STIBP on GET_SUPPORTED

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Message-ID: <CALMp9eQFqs7w81gWYc=z_YXaBGO-4Nr3_M=rd3eev4KP-U-nGA@mail.gmail.com>
Date:   Wed, 5 Dec 2018 14:47:03 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Eduardo Habkost <ehabkost@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH] kvm: x86: Report STIBP on GET_SUPPORTED_CPUID

On Wed, Dec 5, 2018 at 2:02 PM Konrad Rzeszutek Wilk
<konrad.wilk@...cle.com> wrote:
>
> On Wed, Dec 05, 2018 at 05:19:56PM -0200, Eduardo Habkost wrote:
> > Months ago, we have added code to allow direct access to MSR_IA32_SPEC_CTRL
> > to the guest, which makes STIBP available to guests.  This was implemented
> > by commits d28b387fb74d ("KVM/VMX: Allow direct access to
> > MSR_IA32_SPEC_CTRL") and b2ac58f90540 ("KVM/SVM: Allow direct access to
> > MSR_IA32_SPEC_CTRL").
> >
> > However, we never updated GET_SUPPORTED_CPUID to let userspace know that
> > STIBP can be enabled in CPUID.  Fix that by updating
>
> Ooops!
> > kvm_cpuid_8000_0008_ebx_x86_features and kvm_cpuid_7_0_edx_x86_features.
>
> Shouldn't there also be a patch in QEMU to use it? (aka, +stibp).

I don't see how that's relevant here. GET_SUPPORTED_CPUID should
report all of the supported CPUID features regardless of what
particular consumers choose to do with them.