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

[Jim Mattson]

On Wed, Oct 4, 2023 at 12:59 AM Borislav Petkov <bp@xxxxxxxxx> 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.