linux-kernel - [PATCH] KVM: x86/mmu: Add comment on try_cmpxchg64 usage in tdp_mmu_set_spte

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Message-Id: <20230425113932.3148-1-ubizjak@gmail.com>
Date:   Tue, 25 Apr 2023 13:39:32 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        David Matlack <dmatlack@...gle.com>
Subject: [PATCH] KVM: x86/mmu: Add comment on try_cmpxchg64 usage in tdp_mmu_set_spte_atomic

Commit aee98a6838d5 ("KVM: x86/mmu: Use try_cmpxchg64 in
tdp_mmu_set_spte_atomic") removed the comment that iter->old_spte is
updated when different logical CPU modifies the page table entry.
Although this is what try_cmpxchg does implicitly, it won't hurt
if this fact is explicitly mentioned in a restored comment.

Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Sean Christopherson <seanjc@...gle.com>
Cc: David Matlack <dmatlack@...gle.com>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 7c25dbf32ecc..5d126b015086 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -655,8 +655,16 @@ static inline int tdp_mmu_set_spte_atomic(struct kvm *kvm,
 	 * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and
 	 * does not hold the mmu_lock.
 	 */
-	if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte))
+	if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte)) {
+		/*
+		 * The page table entry was modified by a different logical
+		 * CPU. In this case the above try_cmpxchg updates
+		 * iter->old_spte with the current value, so the caller
+		 * operates on fresh data, e.g. if it retries
+		 * tdp_mmu_set_spte_atomic().
+		 */
 		return -EBUSY;
+	}
 
 	__handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
 			      new_spte, iter->level, true);
-- 
2.40.0