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

[Paolo Bonzini]

On Fri, May 10, 2024 at 3:47 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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.

> > +      */
> > +     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.

> Which is much, much more self-explanatory.

Both more self-explanatory and more correct.

Paolo