Re: [PATCH v1 06/23] KVM: VMX: Defer enabling FRED MSRs save/load until after set CPUID

[Sean Christopherson]

On Tue, Nov 14, 2023, Xin3 Li wrote:
> > Implement what in a different way?  The VMCS fields and FRED are architectural.
> > The internal layout of the VMCS is uarch specific, but the encodings and semantics
> > absolutely cannot change without breaking software.  And if Intel does something
> > asinine like make a control active-low then we have far, far bigger problems.
> 
> I should have made it clear that I wasn't talking at the ISA level.  And
> of course CPU uarch implementations should be transparent to software.
> 
> I mean a CPU uarch could choose to check the activation bit in the VM exit
> controls first and then decide whether to load the 2nd VM exit controls.
> While if resources allow, a CPU uarch could always load the 2nd VM exit
> controls.

And why does that matter?  Loading a field speculatively/out-of-order is fine,
consuming it when it architecturally is supposed to be ignored is not.