[PATCH v2 7/8] KVM: arm64: Make VNCR invalidation participate in MMU invalidation retry

[Marc Zyngier <maz@kernel.org>]

A VNCR TLB invalidation can occur on one vcpu while another vcpu is
faulting in this same page. Without correctly handling this, we can
end up with the following scenario:

- vcpu A walks the PTs to translate VNCR
- before vcpu A is able to grab the MMU lock to insert the TLB,
  vcpu B updates the S1 PTs with an invalid entry, and issues
  a TLBI S1E2 for this VA
- vcpu A inserts the TLB for something that is now invalid

This isn't a new problem, and we manage S2 by having the MMU notifier
to bump up mmu_invalidate_seq on invalidation so that the fault can be
replayed.

We can perform something similar here, and extend invalidate_vncr_va() to
update the same counter, clearly indicating that the context has
changed under our feet. This is safe as the invalidation always happen
while holding the MMU lock for write, and that we sample the sequence
number before walking S1.

Fixes: 4ffa72ad8f37e ("KVM: arm64: nv: Add S1 TLB invalidation primitive for VNCR_EL2")
Reported-by: sashiko-bot@kernel.org
Link: https://lore.kernel.org/r/20260801130454.5D9F11F00AC4@smtp.kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
---
 arch/arm64/kvm/nested.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index cf0d45059edbd..550c9bd3dbe7d 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -1058,6 +1058,12 @@ static void kvm_invalidate_vncr_ipa(struct kvm *kvm, u64 start, u64 end)
 	if (!kvm_has_feat(kvm, ID_AA64MMFR4_EL1, NV_frac, NV2_ONLY))
 		return;
 
+	/*
+	 * Note that invalidating the VNCR on the back of an MMU notifier
+	 * doesn't require messing with the invalidation counter for a
+	 * parallel walk. The notifier itself will have bumped the counter,
+	 * making sure we rewalk.
+	 */
 	kvm_for_each_vncr_tlb(i, vcpu, vt, kvm)
 		if (vncr_tlb_intersects(vt, vt->wr.pa, start, end - start))
 			invalidate_vncr(vt);
@@ -1085,6 +1091,15 @@ static void invalidate_vncr_va(struct kvm *kvm,
 
 	lockdep_assert_held_write(&kvm->mmu_lock);
 
+	/*
+	 * We might be performing a parallel S1 walk, so bump up the
+	 * invalidation counter even in the absence of an actual VNCR TLB
+	 * invalidation, as this could indicate that the guest has gone
+	 * through a BBM sequence.
+	 */
+	kvm->mmu_invalidate_seq++;
+	smp_wmb();
+
 	kvm_for_each_vncr_tlb(i, vcpu, vt, kvm) {
 		switch (scope->type) {
 		case TLBI_ALL:
@@ -1419,15 +1434,15 @@ static int kvm_translate_vncr(struct kvm_vcpu *vcpu, bool *is_gmem)
 
 	va =  read_vncr_el2(vcpu);
 
+	mmu_seq = vcpu->kvm->mmu_invalidate_seq;
+	smp_rmb();
+
 	ret = __kvm_translate_va(vcpu, &vt->wi, &vt->wr, va);
 	if (ret)
 		return ret;
 
 	write_fault = kvm_is_write_fault(vcpu);
 
-	mmu_seq = vcpu->kvm->mmu_invalidate_seq;
-	smp_rmb();
-
 	gfn = vt->wr.pa >> PAGE_SHIFT;
 	memslot = gfn_to_memslot(vcpu->kvm, gfn);
 	if (!memslot) {
-- 
2.47.3