Re: [PATCH 2/4] KVM: x86/tdp_mmu: Merge the prefetch into the is_access_allowed() check

[Sean Christopherson <seanjc@google.com>]

On Sat, Feb 08, 2025, Yan Zhao wrote:
> On Fri, Feb 07, 2025 at 07:03:46AM -0800, Sean Christopherson wrote:
> > On Fri, Feb 07, 2025, Yan Zhao wrote:
> > > Merge the prefetch check into the is_access_allowed() check to determine a
> > > spurious fault.
> > > 
> > > In the TDP MMU, a spurious prefetch fault should also pass the
> > > is_access_allowed() check.
> > 
> > How so? 
> > 
> >   1. vCPU takes a write-fault on a swapped out page and queues an async #PF
> >   2. A different task installs a writable SPTE
> >   3. A third task write-protects the SPTE for dirty logging
> >   4. Async #PF handler faults in the SPTE, encounters a read-only SPTE for its
> >      write fault.
> > 
> > KVM shouldn't mark the gfn as dirty in this case.
> Hmm, but when we prefetch an entry, if a gfn is not write-tracked, it allows to
> mark the gfn as dirty, just like when there's no existing SPTE, a prefetch fault
> also marks a gfn as dirty.

Yeah, but there's a difference between installing a SPTE and overwriting a SPTE.

> If a gfn is write-tracked, make_spte() will not grant write-permission to make
> the gfn dirty.
> 
> However, I admit that making the new SPTE as not-accessed again is not desired.
> What about below?
> 
> @@ -983,7 +983,7 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
>                 return RET_PF_RETRY;
> 
>         if (is_shadow_present_pte(iter->old_spte) &&
> -           is_access_allowed(fault, iter->old_spte) &&
> +           (fault->prefetch || is_access_allowed(fault, iter->old_spte)) &&
>             is_last_spte(iter->old_spte, iter->level))
>                 return RET_PF_SPURIOUS;

Works for me.