Re: [PATCH v6 2/5] KVM: x86: Add IBPB support

[Konrad Rzeszutek Wilk]

On Fri, Feb 02, 2018 at 08:31:27PM +0000, David Woodhouse wrote:
> On Fri, 2018-02-02 at 15:28 -0500, Konrad Rzeszutek Wilk wrote:
> > 
> > > 
> > > No. The AMD feature bits give us more fine-grained support for exposing
> > > IBPB or IBRS alone, so we expose those bits on Intel too.
> > 
> > But but.. that runs smack against the idea of exposing a platform that
> > is as close to emulating the real hardware as possible.
> > 
> > As in I would never expect an Intel CPU to expose the IBPB on the 0x8000_0008
> > leaf. Hence KVM (nor any hypervisor) should not do it either.
> > 
> > Unless Intel is doing it? Did I miss a new spec update?
> 
> Are you telling me there's no way you can infer from CPUID that you're
> running in a hypervisor?

That is not what I am saying. The CPUIDs 0x40000000 ... 0x400000ff
are reserved for hypervisor usage. The SDM is pretty clear about it.

The Intel SDM and the AMD equivalant are pretty clear about what the
other leafs should have on its platform.

[5 minutes later]

And I am eating my words here. 

CPUID.80000008 shows how MAXPHYSADDR is used (on the Intel SDM).

Never mind the noise.