Re: [PATCH v15 21/23] KVM: MMU: Disable fast path for private memslots

[Michael Roth <michael.roth@amd.com>]

On Fri, May 10, 2024 at 03:50:26PM +0200, Paolo Bonzini wrote:
> On Fri, May 10, 2024 at 3:47 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > > +      * Since software-protected VMs don't have a notion of a shared vs.
> > > +      * private that's separate from what KVM is tracking, the above
> > > +      * KVM_EXIT_MEMORY_FAULT condition wouldn't occur, so avoid the
> > > +      * special handling for that case for now.
> >
> > Very technically, it can occur if userspace _just_ modified the attributes.  And
> > as I've said multiple times, at least for now, I want to avoid special casing
> > SW-protected VMs unless it is *absolutely* necessary, because their sole purpose
> > is to allow testing flows that are impossible to excercise without SNP/TDX hardware.
> 
> Yep, it is not like they have to be optimized.

Ok, I thought there were maybe some future plans to use sw-protected VMs
to get some added protections from userspace. But even then there'd
probably still be extra considerations for how to handle access tracking
so white-listing them probably isn't right anyway.

I was also partly tempted to take this route because it would cover this
TDX patch as well:

  https://lore.kernel.org/lkml/91c797997b57056224571e22362321a23947172f.1705965635.git.isaku.yamahata@intel.com/

and avoid any weirdness about checking kvm_mem_is_private() without
checking mmu_invalidate_seq, but I think those cases all end up
resolving themselves eventually and added some comments around that.

> 
> > > +      */
> > > +     if (kvm_slot_can_be_private(fault->slot) &&
> > > +         !(IS_ENABLED(CONFIG_KVM_SW_PROTECTED_VM) &&
> > > +           vcpu->kvm->arch.vm_type == KVM_X86_SW_PROTECTED_VM))
> >
> > Heh, !(x && y) kills me, I misread this like 4 times.
> >
> > Anyways, I don't like the heuristic.  It doesn't tie the restriction back to the
> > cause in any reasonable way.  Can't this simply be?
> >
> >         if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)
> >                 return false;
> 
> You beat me to it by seconds. And it can also be guarded by a check on
> kvm->arch.has_private_mem to avoid the attributes lookup.

I re-tested with things implemented this way and everything seems to
look good. It's not clear to me whether this would cover the cases the
above-mentioned TDX patch handles, but no biggie if that's still needed.

The new version of the patch is here:

  https://github.com/mdroth/linux/commit/39643f9f6da6265d39d633a703c53997985c1208

And I've updated my branches with to replace the old patch and also
incorporate Sean's suggestions for patch 22:

  https://github.com/mdroth/linux/commits/snp-host-v15c3-unsquashed

and have them here with things already squashed in/relocated:

  https://github.com/mdroth/linux/commits/snp-host-v15c3

Thanks for the feedback Sean, Paolo.

-Mike
  
> 
> > Which is much, much more self-explanatory.
> 
> Both more self-explanatory and more correct.
> 
> Paolo
> 
>