Re: [PATCH v4 11/19] x86/cpu: Print VMX flags in /proc/cpuinfo using VMX_FEATURES_*

Sean Christopherson
2019-12-12T18:34:43Z

On Thu, Dec 12, 2019 at 08:04:19PM +0200, Liran Alon wrote:
>
>
> > On 12 Dec 2019, at 19:57, Jim Mattson <jmattson@google.com> wrote:
> >
> > On Thu, Dec 12, 2019 at 9:53 AM Liran Alon <liran.alon@oracle.com> wrote:
> >
> >> Why should CPU VMX features be treated differently than standard CPUID deduced features?
> >
> > Do we have the right Intel people on the recipient list to answer this
> > question? Presumably, Intel felt that this information should be
> > available in supervisor mode only.
> >
> > Sean?
>
> Good question. Probably because it just makes sense that Ring3 will never need to use
> this info as all VMX instructions are privileged. i.e. Can only be executed in Ring0.

I highly doubt ring0 vs. ring3 was a motivating factor. I suspect the MSR
interface is primarily driven by VMX's allowed-0 vs. allowed-1 behavior,
which would be awkward to encode in CPUID. Reporting via MSR also likely
provided more flexibility for updating/fixing CPU behavior, e.g. patching
the RDMSR hook is likely far easier than patching CPUID.

Even if the architects intended the information to be supervisor-only,
that's just their opinion, no?

> De-facto in KVM we have discovered this assumption to be problematic BTW,
> as KVM created an interface to query VMX MSRs values to properly define the requested
> vCPU model. :P (See kvm_get_msr_feature())