linux-kernel - Re: [PATCH] KVM: x86: work around QEMU issue with synthetic CPUID leaves

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Message-ID: <a3868519e825075169e8f53620f18bdea9c6b5de.camel@redhat.com>
Date:   Mon, 02 May 2022 09:25:41 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: work around QEMU issue with synthetic CPUID
 leaves

On Sun, 2022-05-01 at 19:37 +0200, Paolo Bonzini wrote:
> On 5/1/22 13:16, Maxim Levitsky wrote:
> > > +		 * However, only do it if the host has CPUID leaf 0x8000001d.
> > > +		 * QEMU thinks that it can query the host blindly for that
> > > +		 * CPUID leaf if KVM reports that it supports 0x8000001d or
> > > +		 * above.  The processor merrily returns values from the
> > > +		 * highest Intel leaf which QEMU tries to use as the guest's
> > > +		 * 0x8000001d.  Even worse, this can result in an infinite
> > > +		 * loop if said highest leaf has no subleaves indexed by ECX.
> > 
> > Very small nitpick: It might be useful to add a note that qemu does this only for the
> > leaf 0x8000001d.
> 
> Yes, it's there: "QEMU thinks that it can query the host blindly for 
> that CPUID leaf", "that" is 0x8000001d in the previous sentence.

Yes I see it, but it doesn't state that qemu doesn't do this to other leaves in the affected range.

I had to check the qemu source to verify this to be sure that checking for 0x8000001d
is enough.

Just a tiny minor nitpick though.

Best regards,
	Maxim Levitsky

> 
> Paolo
>