linux-kernel - Re: [PATCH] x86: kvm: Restrict X86_FEATURE_VMMCALL to x86

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Message-ID: <CAMzpN2jY7AC2Kmp7rr+cP2Xynx66fWnzEXs9_JhbNpsqix0_Vg@mail.gmail.com>
Date:   Wed, 1 Aug 2018 11:21:36 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     whiteheadm@....org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: kvm: Restrict X86_FEATURE_VMMCALL to x86_64 platform

On Tue, Jul 31, 2018 at 9:00 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 31/07/2018 14:57, tedheadster wrote:
> >>
> >> This shouldn't be necessary; for systems that don't have virtualization
> >> extensions, the comment explains why setting X86_FEATURE_VMMCALL is safe.
> >>
> >> But it is also wrong, because you can run a 32-bit kernel as a guest on
> >> a 64-bit processor, and then it should set X86_FEATURE_VMMCALL because
> >> the processor has the vmmcall instruction and not Intel's vmcall.
> >>
> >
> > Paolo,
> >   I'm running this on a bare metal machine (no virtualization) with a
> > 32-bit AMD i486 class cpu. Should the feature be showing up in
> > /proc/cpuinfo under the 'flags' line? It does on my machine, and it
> > looked wrong to me.
>
> It's a bit silly, but it's not particularly wrong.

Why is there even a specific feature flag for VMMCALL?  Isn't
X86_FEATURE_SVM sufficient to differentiate which opcode to use?

--
Brian Gerst