Re: [PATCH 04/16] KVM: x86/mmu: Add address conversion functions for TDX shared bit of GPA

Edgecombe, Rick P
2024-05-16T00:35:41Z

On Thu, 2024-05-16 at 12:25 +1200, Huang, Kai wrote:
>
>
> On 16/05/2024 12:19 pm, Edgecombe, Rick P wrote:
> > On Thu, 2024-05-16 at 12:12 +1200, Huang, Kai wrote:
> > >
> > > I don't have strong objection if the use of kvm_gfn_shared_mask() is
> > > contained in smaller areas that truly need it.  Let's discuss in
> > > relevant patch(es).
> > >
> > > However I do think the helpers like below makes no sense (for SEV-SNP):
> > >
> > > +static inline bool kvm_is_private_gpa(const struct kvm *kvm, gpa_t gpa)
> > > +{
> > > +       gfn_t mask = kvm_gfn_shared_mask(kvm);
> > > +
> > > +       return mask && !(gpa_to_gfn(gpa) & mask);
> > > +}
> >
> > You mean the name? SNP doesn't have a concept of "private GPA" IIUC. The C
> > bit
> > is more like an permission bit. So SNP doesn't have private GPAs, and the
> > function would always return false for SNP. So I'm not sure it's too
> > horrible.
>
> Hmm.. Why SNP doesn't have private GPAs?  They are crypto-protected and
> KVM cannot access directly correct?

I suppose a GPA could be pointing to memory that is private. But I think in SNP
it is more the memory that is private. Now I see more how it could be confusing.

>
> >
> > If it's the name, can you suggest something?
>
> The name make sense, but it has to reflect the fact that a given GPA is
> truly private (crypto-protected, inaccessible to KVM).

If this was a function that tested whether memory is private and took a GPA, I
would call it is_private_mem() or something. Because it's testing the memory and
takes a GPA, not testing the GPA. Usually a function name should be about what
the function does, not what arguments it takes.

I can't think of a better name, but point taken that it is not ideal. I'll try
to think of something.