Re: [PATCH 25/25] KVM: x86: Add CPUID bits missing from KVM_GET_SUPPORTED_CPUID

[Sean Christopherson]

On Tue, Aug 13, 2024, Rick P Edgecombe wrote:
> On Tue, 2024-08-13 at 19:34 +0800, Chao Gao wrote:
> > Mandating that all fixed-1 bits be supported by KVM would be a burden for both
> > KVM and the TDX module: the TDX module couldn't add any fixed-1 bits until KVM
> > supports them, and 
> 
> > KVM shouldn't drop any feature that was ever a fixed-1 bit
> > in any TDX module.
> 
> Honest question...can/does this happen for normal VMs? KVM dropping support for
> features?

Almost never.  KVM still supports Intel CPUs without virtual NMI support, which
IIRC was something like one SKU of Yonah that was 32-bit only.  Keeping backwards
compability is annoying from time to time, but it's generally not that much of a
maintenance burden.  The only CPUs I really wish had never existed are those that
have EPT without A/D bits.  Other than that, maintaining support for old CPUs
doesn't hinder us too much.

> I think I recall even MPX getting limped along for backward compatibility reasons.

Yep, KVM still supports virtualizing MPX.