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

[Yan Zhao]

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.
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;