[PATCH 3/4] KVM: x86/mmu: Top-up memory caches when retrying "map private PFN"

[Sean Christopherson]

When mapping a private PFN in TDX's post-populate callback, top-up the
memory caches on every attempt to map the PFN to harden against bugs in the
map flow that could consume cache entries even if mapping ultimately fails.
E.g. as pointed out by Sashiko, the in-progress Dynamic PAMT support could
consume PAMT cache entries on TDX-Module lock contention.

Harden KVM even though consuming an entry on failure is considered a KVM
bug, as retry is uncommon, top-up is "free" if there's no work to be done,
and populating a TDX guest's memory is a slow path, i.e. there's no
meaningful downside to the hardening.

Reported-by: Sashiko Bot <sashiko-bot@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/20260718061050.E17B01F000E9@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index c6cac893cbad..379f570ef04f 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5184,10 +5184,6 @@ int kvm_tdp_mmu_map_private_pfn(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
 	if (kvm_gfn_is_write_tracked(kvm, fault.slot, fault.gfn))
 		return -EPERM;
 
-	r = mmu_topup_memory_caches(vcpu, false);
-	if (r)
-		return r;
-
 	do {
 		if (signal_pending(current))
 			return -EINTR;
@@ -5199,6 +5195,10 @@ int kvm_tdp_mmu_map_private_pfn(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
 		if (r)
 			return r;
 
+		r = mmu_topup_memory_caches(vcpu, false);
+		if (r)
+			return r;
+
 		cond_resched();
 
 		guard(read_lock)(&kvm->mmu_lock);
-- 
2.55.0.679.g6767b8d81c-goog