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

Sean Christopherson
2020-03-05T19:25:34Z

On Thu, Mar 05, 2020 at 10:43:51AM -0800, Jim Mattson wrote:
> On Wed, Mar 4, 2020 at 5:34 PM Sean Christopherson
> <sean.j.christopherson@intel.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!

Hmm, is it actually needed here? KVM doesn't advertise support for that
range in KVM_GET_SUPPORTED_CPUID. That's also why I limited the Centaur
range to vendor==CENTAUR, as KVM_GET_SUPPORTED_CPUID enumerates the
Centaur range if and only if the host CPU is Centaur.