Re: [PATCH v5 1/5] KVM: TDX: Check unsupported CPUID bits against the TDX module's subleaf

[Sean Christopherson <seanjc@google.com>]

On Sat, Oct 17, 2026, Binbin Wu wrote:
> On 10/16/2026 11:25 PM, Sean Christopherson wrote:
> > On Fri, Oct 16, 2026, Binbin Wu wrote:
> > @@ -2386,7 +2390,7 @@ static int setup_tdparams_cpuids(struct kvm_cpuid2 *cpuid,
> >                 if (!entry)
> >                         continue;
> >  
> > -               if (tdx_unsupported_cpuid(entry))
> > +               if (tdx_is_unsupported_cpuid_entry(entry))
> >                         return -EINVAL;
> >  
> >                 copy_cnt++;
> > 
> > Can you test the attached patches?  If they boot TDX with QEMU, I'll go ahead
> > and post.
> 
> Patch 3 "KVM: TDX: Don't zero non-significant CPUID entry indices" breaks QEMU.
> 
> QEMU uses zero as index for cpuid entries when index is not significant.

Drat.  __do_cpuid_func() => do_host_cpuid() also zeros the index for
KVM_GET_SUPPORTED_CPUID, so the ABI is definitely baked at this point. 

> And in QEMU, the lookup function doesn't consult the flags at all.
> 
> struct kvm_cpuid_entry2 *cpuid_find_entry(struct kvm_cpuid2 *cpuid,
>                                           uint32_t function,
>                                           uint32_t index)
> {
>     int i;
>     for (i = 0; i < cpuid->nent; ++i) {
>         if (cpuid->entries[i].function == function &&
>             cpuid->entries[i].index == index) {

Bad QEMU, bad!  :-)

>             return &cpuid->entries[i];
>         }
>     }
>     /* not found: */
>     return NULL;
> }
> 
> If KVM doesn't zero the index for the non-significant, QEMU cannot find
> any entry for non-significant entries returned by KVM_TDX_CAPABILITIES.

I'll opportunistically add a comment to call out that even though the index
should be meaningless, it has become ABI that userspace relies on.

> And 0x80000008 is used to pass GPAW, and KVM_TDX_INIT_VM fails when QEMU
> doesn't pass it.
> 
> After dropping patch 3, QEMU can boot TD.
> Also our internal TDX CI cases passed.

Thanks much!