linux-kernel - Re: [PATCH v2 4/7] KVM: x86: Fix CPUID range checks for Hypervisor and Centaur classes

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Date:   Tue, 10 Mar 2020 10:23:44 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, Pu Wen <puwen@...on.cn>
Subject: Re: [PATCH v2 4/7] KVM: x86: Fix CPUID range checks for Hypervisor
 and Centaur classes

On Tue, Mar 10, 2020 at 10:10 AM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Fri, Mar 06, 2020 at 10:03:37AM +0100, Paolo Bonzini wrote:
> > On 05/03/20 22:51, Sean Christopherson wrote:
> > >> Ah. So cross-vendor CPUID specifications are not supported?
> > > Cross-vendor CPUID is sort of allowed?  E.g. this plays nice with creating
> > > a Centaur CPU on an Intel platform.  My interpretation of GET_SUPPORTED...
> > > is that KVM won't prevent enumerating what you want in CPUID, but it only
> > > promises to correctly support select leafs.
> >
> > But in practice does this change anything?  If the vendor is not Centaur
> > it's unlikely that there is a 0xc0000000 leaf.  The 0x80000000 bound is
> > certainly not going to be at 0xc0000000 or beyond, and likewise to 0xc0000000
> > bound is not going to be at 0xd0000000 or beyond.  So I'm not sure if
> > anything is lost from this simplification:
>
> Probably not?  But in the unlikely scenario that Intel wants to add a CPUID
> leaf above 0xc0000000, I don't want to have to explain that it might cause
> problems for KVM guests because I added code to emulate (alleged) Centaur
> behavior for virtual Intel CPUs.

And there is some precedent for that, with the 0x20000000 leaves.