Re: [PATCH v14 21/22] KVM: selftests: Add ucall support for TDX

[Xiaoyao Li]

On 8/28/2026 10:06 AM, Peter Fang wrote:
> On Mon, Aug 17, 2026 at 04:38:49PM +0800, Xiaoyao Li wrote:
> > > +
> > > +static u8 vm_type;
> > > +static gpa_t ucall_mmio_gpa;
> > > +
> > > +void ucall_arch_init(struct kvm_vm *vm, gpa_t mmio_gpa)
> > > +{
> > > +	vm_type = vm->type;
>
> vm_type is u8 but vm->type is unsigned long
>
> > > +	sync_global_to_guest(vm, vm_type);
> >
> > It works and it looks simple. But we have the architectural approach to test
> > if a guest is TD guest, by checking the CPUID 0x21.
> >
> > Since checking CPUID 0x21 is not complex, and as a bonus it can help test if
> > TDX module behaves correctly for CPUID leaf 0x21, I think we should switch
> > to use CPUID 0x21 to check if it is TDX VM in guest code?
>
> Hmm... This makes me wonder if vm->arch.s_bit below could be replaced
> with the same architectural approach. GPAW is available through
> TDG.VP.INFO or the initial RBX value. This does require a bit more
> plumbing though.

I'm afraid not. Because below is host code, and vm->arch.s_bit is not 
used in guest code.

Or are suggesting something like dropping the

	if (is_tdx_vm(vm)) {
		ucall_mmio_gpa = UCALL_MMIO_GPA | vm->arch.s_bit;
		sync_global_to_guest(vm, ucall_mmio_gpa);
	}

entirely and use below hardcoded value instead in guest code?

	UCALL_MMIO_GPA | 1 << (GPAW - 1)

> > > +	if (is_tdx_vm(vm)) {
> > > +		ucall_mmio_gpa = UCALL_MMIO_GPA | vm->arch.s_bit;
> >
> > So the passed-in @mmio_gpa is not used. Why cannot use the mmio_gpa,
> >
> >    slot0->region.guest_phys_addr + slot0->region.memory_size
> >
> > chosen by __vm_create()?
>
> Looks like this was discussed previously [1]. Maybe at least a comment
> here would help?

yeah. A comment surely will help.

> [1] https://lore.kernel.org/all/ajGaMb6Lrk3OS8B4@xxxxxxxxxx/
>
> >