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

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Date:   Thu, 5 Mar 2020 10:43:51 -0800
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 Wed, Mar 4, 2020 at 5:34 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> Rework the masking in the out-of-range CPUID logic to handle the
> Hypervisor sub-classes, as well as the Centaur class if the guest
> virtual CPU vendor is Centaur.
>
> Masking against 0x80000000 only handles basic and extended leafs, which
> results in Hypervisor range checks being performed against the basic
> CPUID class, and Centuar range checks being performed against the
> Extended class.  E.g. if CPUID.0x40000000.EAX returns 0x4000000A and
> there is no entry for CPUID.0x40000006, then function 0x40000006 would
> be incorrectly reported as out of bounds.
>
> While there is no official definition of what constitutes a class, the
> convention established for Hypervisor classes effectively uses bits 31:8
> as the mask by virtue of checking for different bases in increments of
> 0x100, e.g. KVM advertises its CPUID functions starting at 0x40000100
> when HyperV features are advertised at the default base of 0x40000000.
>
> The bad range check doesn't cause functional problems for any known VMM
> because out-of-range semantics only come into play if the exact entry
> isn't found, and VMMs either support a very limited Hypervisor range,
> e.g. the official KVM range is 0x40000000-0x40000001 (effectively no
> room for undefined leafs) or explicitly defines gaps to be zero, e.g.
> Qemu explicitly creates zeroed entries up to the Cenatur and Hypervisor
> limits (the latter comes into play when providing HyperV features).
>
> The bad behavior can be visually confirmed by dumping CPUID output in
> the guest when running Qemu with a stable TSC, as Qemu extends the limit
> of range 0x40000000 to 0x40000010 to advertise VMware's cpuid_freq,
> without defining zeroed entries for 0x40000002 - 0x4000000f.
>
> Note, documentation of Centaur/VIA CPUs is hard to come by.  Designating
> 0xc0000000 - 0xcfffffff as the Centaur class is a best guess as to the
> behavior of a real Centaur/VIA CPU.

Don't forget Transmeta's CPUID range at 0x80860000 through 0x8086FFFF!