diff options
| author | Ben Gardon <bgardon@google.com> | 2021-04-01 16:37:30 -0700 |
|---|---|---|
| committer | Paolo Bonzini <pbonzini@redhat.com> | 2021-04-19 09:05:25 -0400 |
| commit | fb10129335ca6cc7a229226b03f54394757d773d (patch) | |
| tree | d52fd20442893820e085ba32230b948ca2a7c3e4 /arch/x86 | |
| parent | 11cccf5c04721c8a08f9d72a1a5e7281a4041d86 (diff) | |
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 <bgardon@google.com>
Message-Id: <20210401233736.638171-8-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'arch/x86')
| -rw-r--r-- | arch/x86/kvm/mmu/tdp_mmu.c | 11 | ||||
| -rw-r--r-- | arch/x86/kvm/mmu/tdp_mmu.h | 13 |
2 files changed, 9 insertions, 15 deletions
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); diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h index 0fc9061ff2d8..25268c4ba03b 100644 --- a/arch/x86/kvm/mmu/tdp_mmu.h +++ b/arch/x86/kvm/mmu/tdp_mmu.h @@ -7,17 +7,10 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu); -static inline void kvm_tdp_mmu_get_root(struct kvm *kvm, - struct kvm_mmu_page *root) +__must_check static inline bool kvm_tdp_mmu_get_root(struct kvm *kvm, + struct kvm_mmu_page *root) { - lockdep_assert_held_write(&kvm->mmu_lock); - - /* - * This should never fail since roots are removed from the roots - * list under the MMU write lock when their reference count falls - * to zero. - */ - refcount_inc_not_zero(&root->tdp_mmu_root_count); + return refcount_inc_not_zero(&root->tdp_mmu_root_count); } void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root); |