Re: [PATCH v4 13/18] KVM: guest_memfd: Pass GPA, not GFN, to prepare() hook

[Ackerley Tng]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

> On Thu, Jul 09, 2026, Ackerley Tng wrote:
>> Sean Christopherson <seanjc@xxxxxxxxxx> writes:
>>
>> > Pass a GPA instead of a GFN to kvm_arch_gmem_prepare() so that the GFN/GPA
>> > can be made optional in the future, i.e. so that guest_memfd can pass
>> > INVALID_GPA.  This will allow reworking the hook into a general .convert()
>>
>> How about defining a INVALID_GFN instead of using gpa just so
>> INVALID_GPA can be passed? The SNP side takes a gfn, so it seems like
>> the 2 PAGE_SHIFTs are extra work.
>
> They are, but I am very hesitant to define INVALID_GFN as I'm worried about the
> potential for subtle bugs if both INVALID_GPA and INVALID_GFN exist.  E.g. I
> almost made a goof in this exact series where I was going to do:
>
> 	gfn_t gfn = gpa_to_gfn(svm->sev_es.snp_guest_vmsa_gpa);
>
> 	...
>
> 	if (gfn == INVALID_GFN)

When you did this check INVALID_GFN was defined as (~0) too I suppose...

> 		return;
>
> Where snp_guest_vmsa_gpa is set to INVALID_GPA.
>
> I suppose we could define INVALID_GFN to gpa_to_gfn(INVALID_GPA), but that feels
> weird, and I feel more comfortable having magic invalid values in only one domain.

Maybe INVALID_GPA and INVALID_GFN should be defined such that both
gpa_to_gfn and gfn_to_gpa are correct for both. Isn't the relationship
between gpa and gfn always supposed to be always a fixed shift apart?

When a GPA is passed to the arch function, it almost feels like now the
arch should be validating PAGE_ALIGNED(gpa).

Mentioned this off-list, another thing that seems incongruent is that
one parameter is in the address domain (gpa) and the other is in the
page frame domain (pfn).