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

[Sean Christopherson]

On Wed, Aug 14, 2024, Chao Gao wrote:
> On Tue, Aug 13, 2024 at 11:14:31PM +0800, Xiaoyao Li wrote:
> >On 8/13/2024 7:34 PM, Chao Gao wrote:
> >> I think adding new fixed-1 bits is fine as long as they don't break KVM, i.e.,
> >> KVM shouldn't need to take any action for the new fixed-1 bits, like
> >> saving/restoring more host CPU states across TD-enter/exit or emulating
> >> CPUID/MSR accesses from guests
> >
> >I disagree. Adding new fixed-1 bits in a newer TDX module can lead to a
> >different TD with same cpu model.
> 
> The new TDX module simply doesn't support old CPU models.

What happens if the new TDX module is needed to fix a security issue?  Or if a
customer wants to support a heterogenous migration pool, and older (physical)
CPUs don't support the feature?  Or if a customer wants to continue hosting
existing VM shapes on newer hardware?

> QEMU can report an error and define a new CPU model that works with the TDX
> module. Sometimes, CPUs may drop features;

Very, very rarely.  And when it does happen, there are years of warning before
the features are dropped.

> this may cause KVM to not support some features and in turn some old CPU
> models having those features cannot be supported.  is it a requirement for
> TDX modules alone that old CPU models must always be supported?

Not a hard requirement, but a pretty firm one.  There needs to be sane, reasonable
behavior, or we're going to have problems.