From fb10129335ca6cc7a229226b03f54394757d773d Mon Sep 17 00:00:00 2001 From: Ben Gardon Date: Thu, 1 Apr 2021 16:37:30 -0700 Subject: KVM: x86/mmu: handle cmpxchg failure in kvm_tdp_mmu_get_root To reduce dependence on the MMU write lock, don't rely on the assumption that the atomic operation in kvm_tdp_mmu_get_root will always succeed. By not relying on that assumption, threads do not need to hold the MMU lock in write mode in order to take a reference on a TDP MMU root. In the root iterator, this change means that some roots might have to be skipped if they are found to have a zero refcount. This will still never happen as of this patch, but a future patch will need that flexibility to make the root iterator safe under the MMU read lock. Signed-off-by: Ben Gardon Message-Id: <20210401233736.638171-8-bgardon@google.com> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/mmu/tdp_mmu.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) (limited to 'arch/x86/kvm/mmu/tdp_mmu.c') diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c index f55b415d600a..6c1765c9642c 100644 --- a/arch/x86/kvm/mmu/tdp_mmu.c +++ b/arch/x86/kvm/mmu/tdp_mmu.c @@ -88,10 +88,12 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm, next_root = list_first_entry(&kvm->arch.tdp_mmu_roots, typeof(*next_root), link); + while (!list_entry_is_head(next_root, &kvm->arch.tdp_mmu_roots, link) && + !kvm_tdp_mmu_get_root(kvm, next_root)) + next_root = list_next_entry(next_root, link); + if (list_entry_is_head(next_root, &kvm->arch.tdp_mmu_roots, link)) next_root = NULL; - else - kvm_tdp_mmu_get_root(kvm, next_root); if (prev_root) kvm_tdp_mmu_put_root(kvm, prev_root); @@ -161,10 +163,9 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu) /* Check for an existing root before allocating a new one. */ for_each_tdp_mmu_root(kvm, root, kvm_mmu_role_as_id(role)) { - if (root->role.word == role.word) { - kvm_tdp_mmu_get_root(kvm, root); + if (root->role.word == role.word && + kvm_tdp_mmu_get_root(kvm, root)) goto out; - } } root = alloc_tdp_mmu_page(vcpu, 0, vcpu->arch.mmu->shadow_root_level); -- cgit v1.2.3