Re: [PATCH] KVM: TDX: Fix APIC MSR ranges in tdx_has_emulated_msr()

[Sean Christopherson <seanjc@google.com>]

On Sat, Apr 04, 2026, Rick P Edgecombe wrote:
> On Fri, 2026-04-03 at 16:07 -0700, Sean Christopherson wrote:
> > > So... "Reduced #VE" (also called "VE reduction") reduces which things cause
> > > a #VE. The guest opts into it and the TDX module starts behaving
> > > differently. It's kind of grab bag of changes including changing CPUID
> > > behavior, which is another wrinkle. It was intended to fixup guest side TDX
> > > arch issues.
> > 
> > And KVM has no visilibity into which mode the guest has selected?  That's
> > awful.
> 
> Yea, on both accounts. So where we are at with this is, starting to reject
> changes that build on the pattern. We haven't gone so far as to ask for a
> feature to notify the host of the guest opt-ins. But I wouldn't say we have a
> grand design in mind either. If you have any clarity, please feel free to drop a
> quotable.

I got nothing, probably best to just deal with things on a case-by-case basis
unless we end up with a recurring theme.

> > If KVM has no visiblity, then I don't see an option other than for KVM to
> > advertise and emulate what it can at all times, and it becomes the guest's
> > responsibility to not screw up.  I guess it's not really any different from
> > not trying to use MMIO accesses after switching to x2APIC mode.
> 
> Like your diff? Expose any MSRs that might be emulated in the TDX paradigm. But
> don't expose all MSRs that KVM supports.

Yep.