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

Yan Zhao
2024-05-16T05:53:10Z

On Thu, May 16, 2024 at 01:40:41PM +1200, Huang, Kai wrote:
>
>
> On 16/05/2024 1:20 pm, Edgecombe, Rick P wrote:
> > On Thu, 2024-05-16 at 13:04 +1200, Huang, Kai wrote:
> > >
> > > I really don't see difference between ...
> > >
> > >         is_private_mem(gpa)
> > >
> > > ... and
> > >
> > >         is_private_gpa(gpa)
> > >
> > > If it confuses me, it can confuses other people.
> >
> > Again, point taken. I'll try to think of a better name. Please share if you do.
> >
> > >
> > > The point is there's really no need to distinguish the two.  The GPA is
> > > only meaningful when it refers to the memory that it points to.
> > >
> > > So far I am not convinced we need this helper, because such info we can
> > > already get from:
> > >
> > >    1) fault->is_private;
> > >    2) Xarray which records memtype for given GFN.
> > >
> > > So we should just get rid of it.
> >
> > Kai, can you got look through the dev branch a bit more before making the same
> > point on every patch?
> >
> > kvm_is_private_gpa() is used to set PFERR_PRIVATE_ACCESS, which in turn sets
> > fault->is_private. So you are saying we can use these other things that are
> > dependent on it. Look at the other callers too.
>
> Well, I think I didn't make myself clear.
>
> I don't object to have this helper. If it helps, then we can have it.
>
> My objection is the current implementation of it, because it is
> *conceptually* wrong for SEV-SNP.
>
> Btw, I just look at the dev branch.
>
> For the common code, it is used in kvm_tdp_mmu_map() and
> kvm_tdp_mmu_fast_pf_get_last_sptep() to get whether a GPA is private.
>
> As said above, I don't see why we need a helper with the "current
> implementation" (which consults kvm_shared_gfn_mask()) for them. We can
> just use fault->gfn + fault->is_private for such purpose.
What about a name like kvm_is_private_and_mirrored_gpa()?
Only TDX's private memory is mirrored and the common code needs a way to
tell that.

> It is also used in the TDX code like TDX variant handle_ept_violation() and
> tdx_vcpu_init_mem_region(). For them to be honest I don't quite care
> whether a helper is used. We can have a helper if we have multiple callers,
> but this helper should be in TDX code, but not common MMU code.
>