[PATCH v2 06/13] KVM: x86/mmu: Make TDP MMU root refcount atomic

[Ben Gardon]

In order to parallelize more operations for the TDP MMU, make the
refcount on TDP MMU roots atomic, so that a future patch can allow
multiple threads to take a reference on the root concurrently, while
holding the MMU lock in read mode.

Signed-off-by: Ben Gardon <bgar...@google.com>
---

Changelog
v2:
--      Split failure handling for kvm_tdp_mmu_get_root out into a
        seperate commit.

 arch/x86/kvm/mmu/mmu_internal.h |  6 +++++-
 arch/x86/kvm/mmu/tdp_mmu.c      |  4 ++--
 arch/x86/kvm/mmu/tdp_mmu.h      | 10 +++++++---
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 9347d73996b5..f63d0fdb8567 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -50,7 +50,11 @@ struct kvm_mmu_page {
        u64 *spt;
        /* hold the gfn of each spte inside spt */
        gfn_t *gfns;
-       int root_count;          /* Currently serving as active root */
+       /* Currently serving as active root */
+       union {
+               int root_count;
+               refcount_t tdp_mmu_root_count;
+       };
        unsigned int unsync_children;
        struct kvm_rmap_head parent_ptes; /* rmap pointers to parent sptes */
        DECLARE_BITMAP(unsync_child_bitmap, 512);
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 670c5e3ad80e..697ea882a3e4 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -56,7 +56,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct 
kvm_mmu_page *root)
 
        lockdep_assert_held_write(&kvm->mmu_lock);
 
-       if (--root->root_count)
+       if (!refcount_dec_and_test(&root->tdp_mmu_root_count))
                return;
 
        WARN_ON(!root->tdp_mmu_page);
@@ -168,7 +168,7 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu)
        }
 
        root = alloc_tdp_mmu_page(vcpu, 0, vcpu->arch.mmu->shadow_root_level);
-       root->root_count = 1;
+       refcount_set(&root->tdp_mmu_root_count, 1);
 
        list_add(&root->link, &kvm->arch.tdp_mmu_roots);
 
diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
index d4e32ac5f4c9..1ec7914ecff9 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -10,10 +10,14 @@ 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)
 {
-       BUG_ON(!root->root_count);
-       lockdep_assert_held(&kvm->mmu_lock);
+       lockdep_assert_held_write(&kvm->mmu_lock);
 
-       ++root->root_count;
+       /*
+        * 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);
 }
 
 void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root);
-- 
2.31.0.208.g409f899ff0-goog