From 12ced095956a517dc0c8c8f9264e29fd67124302 Mon Sep 17 00:00:00 2001 From: Uros Bizjak Date: Tue, 25 Apr 2023 13:39:32 +0200 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 Cc: Sean Christopherson Cc: David Matlack Signed-off-by: Uros Bizjak Link: https://lore.kernel.org/r/20230425113932.3148-1-ubizjak@gmail.com [sean: extend comment above try_cmpxchg64()] Signed-off-by: Sean Christopherson --- arch/x86/kvm/mmu/tdp_mmu.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c index 0834021..512163d 100644 --- a/arch/x86/kvm/mmu/tdp_mmu.c +++ b/arch/x86/kvm/mmu/tdp_mmu.c @@ -592,7 +592,10 @@ 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. + * does not hold the mmu_lock. On failure, i.e. if a different logical + * CPU modified the SPTE, try_cmpxchg64() 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() */ if (!try_cmpxchg64(sptep, &iter->old_spte, new_spte)) return -EBUSY; -- 2.7.4