[PATCH v2 07/13] KVM: x86/mmu: handle cmpxchg failure in kvm_tdp_mmu_get_root

Ben Gardon
2021-04-01T23:38:31Z

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>
---
arch/x86/kvm/mmu/tdp_mmu.c | 11 ++++++-----
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 697ea882a3e4..886bc170f2a5 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 1ec7914ecff9..f0a26214e999 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);
--
2.31.0.208.g409f899ff0-goog