Re: [PATCH 08/16] KVM: x86/mmu: Bug the VM if kvm_zap_gfn_range() is called for TDX

[Edgecombe, Rick P]

On Wed, 2024-05-15 at 11:09 -0700, Sean Christopherson wrote:
> On Wed, May 15, 2024, Rick P Edgecombe wrote:
> > On Wed, 2024-05-15 at 09:02 -0700, Sean Christopherson wrote:
> > > > Or most specifically, we only need this zapping if we *try* to have
> > > > consistent cache attributes between private and shared. In the
> > > > non-coherent DMA case we can't have them be consistent because TDX
> > > > doesn't support changing the private memory in this way.
> > > 
> > > Huh?  That makes no sense.  A physical page can't be simultaneously mapped
> > > SHARED and PRIVATE, so there can't be meaningful cache attribute aliasing
> > > between private and shared EPT entries.
> > > 
> > > Trying to provide consistency for the GPA is like worrying about having
> > > matching PAT entires for the virtual address in two different processes.
> > 
> > No, not matching between the private and shared mappings of the same page.
> > The
> > whole private memory will be WB, and the whole shared half will honor PAT.
> 
> I'm still failing to see why that's at all interesting.  The non-coherent DMA
> trainwreck is all about KVM worrying about the guest and host having different
> memtypes for the same physical page.

Ok. The split seemed a little odd and special. I'm not sure it's the most
interesting thing in the world, but there was some debate internally about it.

> 
> If the host is accessing TDX private memory, we have far, far bigger problems
> than aliased memtypes.

This wasn't the concern.

>   And so the fact that TDX private memory is forced WB is
> interesting only to the guest, not KVM.

It's just another little quirk in an already complicated solution. They third
thing we discussed was somehow rejecting or not supporting non-coherent DMA.
This seemed simpler than that.