Re: [PATCH] x86: KVM: Add feature flag for AMD's FsGsKernelGsBaseNonSerializing

[Jim Mattson <jmattson@google.com>]

On Thu, Oct 5, 2023 at 9:35 AM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 10/5/23 09:22, Jim Mattson wrote:
> > On Wed, Oct 4, 2023 at 12:59 AM Borislav Petkov <bp@alien8.de> wrote:
> >> On Tue, Oct 03, 2023 at 07:44:51PM -0700, Jim Mattson wrote:
> >>> The business of declaring breaking changes to the architectural
> >>> specification in a CPUID bit has never made much sense to me.
> >> How else should they be expressed then?
> >>
> >> In some flaky PDF which changes URLs whenever the new corporate CMS gets
> >> installed?
> >>
> >> Or we should do f/m/s matching which doesn't make any sense for VMs?
> >>
> >> When you think about it, CPUID is the best thing we have.
> > Every time a new defeature bit is introduced, it breaks existing
> > hypervisors, because no one can predict ahead of time that these bits
> > have to be passed through.
> >
> > I wonder if we could convince x86 CPU vendors to put all defeature
> > bits under a single leaf, so that we can just set the entire leaf to
> > all 1's in KVM_GET_SUPPORTED_CPUID.
>
> I hope I'm not throwing stones from a glass house here...
>
> But I'm struggling to think of cases where Intel has read-only
> "defeature bits" like this one.  There are certainly things like
> MSR_IA32_MISC_ENABLE_FAST_STRING that can be toggled, but read-only
> indicators of a departure from established architecture seems ...
> suboptimal.
>
> It's arguable that TDX changed a bunch of architecture like causing
> exceptions on CPUID and MSRs that never caused exceptions before and
> _that_ constitutes a defeature.  But that's the least of the problems
> for a TDX VM. :)
>
> (Seriously, I'm not trying to shame Intel's x86 fellow travelers here,
>  just trying to make sure I'm not missing something).

Intel's defeature bits that I know of are:

CPUID.(EAX=7,ECX=0):EBX[bit 13] (Haswell) - "Deprecates FPU CS and FPU
DS values if 1."
CPUID.(EAX=7,ECX=0):EBX[bit 6] (Skylake) - "FDP_EXCPTN_ONLY. x87 FPU
Data Pointer updated only on x87 exceptions if 1."