Re: [PATCH v10 09/28] KVM: VMX: Save/restore guest FRED RSP0

[Edgecombe, Rick P]

On Mon, 2026-09-14 at 15:04 +0800, Binbin Wu wrote:
> > Note, an equivalent TDX side implementation is missing. No TDX module
> > with FRED support exists yet so this would be an issue for future
> > platforms. A generic discussion regarding this is ongoing at:
> > https://lore.kernel.org/kvm/20260827031837.2863609-1-binbin.wu@xxxxxxxxxxxxxxx/
> 
> For old KVM versions, we want to defer the opt-in design or backport decision.
> (
> https://lore.kernel.org/kvm/c6b240bd-3c45-4abe-b17c-72a1a7da582f@xxxxxxxxxxxxxx
> m/)
> 
> We may finally choose to backport the TDX CPUID filtering patch series to
> stable kernels. If this patch series lands first, should we add FRED in the
> deny list for TDX as a temporary solution to avoid having some version(s) that
> allow FRED for TDX without FRED support in KVM?

The specific FRED problem starts with FRED host usage + TDX, so I don't think
there is a reason to tie and temporary fixups to VMX FRED support, in any case.
Or were you thinking to blame userspace for not respecting
KVM_GET_SUPPORTED_CPUID, which would change after this series? 

Since there are plenty of released kernels with FRED host support already, I'd
think to wait until we settle the opt-in question. Which doesn't seem like it
should be too much longer. It will be less thrash.