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. Combining these checks to avoid redundancy.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index ab65fd915ef2..5f9e7374220e 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1137,10 +1137,6 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
 	if (WARN_ON_ONCE(sp->role.level != fault->goal_level))
 		return RET_PF_RETRY;
 
-	if (fault->prefetch && is_shadow_present_pte(iter->old_spte) &&
-	    is_last_spte(iter->old_spte, iter->level))
-		return RET_PF_SPURIOUS;
-
 	if (is_shadow_present_pte(iter->old_spte) &&
 	    is_access_allowed(fault, iter->old_spte) &&
 	    is_last_spte(iter->old_spte, iter->level))
-- 
2.43.2

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.

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;

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.