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>

1 files changed, 3 insertions, 10 deletions

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);