Commit fb101293 authored by Ben Gardon's avatar Ben Gardon Committed by Paolo Bonzini
Browse files

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: default avatarBen Gardon <bgardon@google.com>
Message-Id: <20210401233736.638171-8-bgardon@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 11cccf5c

arch/x86/kvm/mmu/tdp_mmu.c

+6 −5
Original line number Diff line number Diff line
@@ -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);
@@ -161,11 +163,10 @@ 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);

	refcount_set(&root->tdp_mmu_root_count, 1);

arch/x86/kvm/mmu/tdp_mmu.h

+3 −10
Original line number Diff line number Diff line
@@ -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,

__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);