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

[Ben Gardon]

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 <bgar...@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