RE: [PATCH v2 04/25] KVM: x86: Mark CR4.FRED as not reserved

On Wed, Feb 07, 2024, Xin Li wrote:

...

The CR4.FRED bit, i.e., CR4[32], is no longer a reserved bit when a guest enumerates FRED, otherwise it is still a reserved bit.

This isn't quite correct, as __cr4_reserved_bits() is used with kvm_cpu_caps too, i.e. to compute CR4 bits that are reserved from the host's perspective. And that matters, because if this check was done _only_ on guest CPUID, then KVM would allow CR4.FRED=1 before all of KVM support is in place.

Ah, that means I didn't dig deep enough.

Thanks! Xin