Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs

[Xin Li <xin@zytor.com>]

On 4/27/2025 8:26 PM, Chao Gao wrote:
>> For KVM, it's just the one MSR, and KVM needs to support save/restore of that MSR
>> no matter what, so supporting it via XSAVE would be more work, a bit sketchy, and
>> create yet another way for userspace to do weird things when saving/restoring vCPU
>> state.
> Agreed. One more issue of including CET_S into KVM_GET/SET_XSAVE{2} is:
> 
> XSAVE UABI buffers adhere to the standard format defined by the SDM, which
> never includes supervisor states. Attempting to incorporate supervisor states
> into UABI buffers would lead to many issues, such as deviating from the
> standard format and the need to define offsets for each supervisor state.

Good point.

FRED RSPs are in the supervisor state and are not defined within the
XSAVE area.  Their save/restore operations need to be explicitly added 
one by one.

Would it be sensible to introduce a KVM supervisor MSR state that
includes new MSRs?

This approach is similar to what KVM has with its CPUID API, allowing
batch MSR set/get operations and improving MSR code sharing.  Since the
FRED state is entirely defined in MSRs (except for CR4.FRED), this
should simplify the context save and restore for FRED.

Thanks!
     Xin