[PATCH v2 08/13] KVM: x86/mmu: Protect the tdp_mmu_roots list with RCU

Ben Gardon
2021-04-01T23:38:29Z

Protect the contents of the TDP MMU roots list with RCU in preparation
for a future patch which will allow the iterator macro to be used under
the MMU lock in read mode.

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

Changelog
v2:
-- add lockdep condition for tdp_mmu_pages_lock to for_each_tdp_mmu_root
-- fix problem with unexported lockdep function
-- updated comments in kvm_host.h

arch/x86/include/asm/kvm_host.h | 21 +++++++---
arch/x86/kvm/mmu/tdp_mmu.c | 69 +++++++++++++++++++--------------
2 files changed, 55 insertions(+), 35 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 99778ac51243..e02e8b8a875b 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1050,25 +1050,36 @@ struct kvm_arch {
bool tdp_mmu_enabled;

/*
- * List of struct kvmp_mmu_pages being used as roots.
+ * List of struct kvm_mmu_pages being used as roots.
* All struct kvm_mmu_pages in the list should have
* tdp_mmu_page set.
- * All struct kvm_mmu_pages in the list should have a positive
- * root_count except when a thread holds the MMU lock and is removing
- * an entry from the list.
+ *
+ * For reads, this list is protected by:
+ * the MMU lock in read mode + RCU or
+ * the MMU lock in write mode
+ *
+ * For writes, this list is protected by:
+ * the MMU lock in read mode + the tdp_mmu_pages_lock or
+ * the MMU lock in write mode
+ *
+ * Roots will remain in the list until their tdp_mmu_root_count
+ * drops to zero, at which point the thread that decremented the
+ * count to zero should removed the root from the list and clean
+ * it up, freeing the root after an RCU grace period.
*/
struct list_head tdp_mmu_roots;

/*
* List of struct kvmp_mmu_pages not being used as roots.
* All struct kvm_mmu_pages in the list should have
- * tdp_mmu_page set and a root_count of 0.
+ * tdp_mmu_page set and a tdp_mmu_root_count of 0.
*/
struct list_head tdp_mmu_pages;

/*
* Protects accesses to the following fields when the MMU lock
* is held in read mode:
+ * - tdp_mmu_roots (above)
* - tdp_mmu_pages (above)
* - the link field of struct kvm_mmu_pages used by the TDP MMU
* - lpage_disallowed_mmu_pages
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 886bc170f2a5..c1d7f6b86870 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -50,6 +50,22 @@ static void tdp_mmu_free_sp(struct kvm_mmu_page *sp)
kmem_cache_free(mmu_page_header_cache, sp);
}

+/*
+ * This is called through call_rcu in order to free TDP page table memory
+ * safely with respect to other kernel threads that may be operating on
+ * the memory.
+ * By only accessing TDP MMU page table memory in an RCU read critical
+ * section, and freeing it after a grace period, lockless access to that
+ * memory won't use it after it is freed.
+ */
+static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
+{
+ struct kvm_mmu_page *sp = container_of(head, struct kvm_mmu_page,
+ rcu_head);
+
+ tdp_mmu_free_sp(sp);
+}
+
void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
{
gfn_t max_gfn = 1ULL << (shadow_phys_bits - PAGE_SHIFT);
@@ -61,11 +77,13 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)

WARN_ON(!root->tdp_mmu_page);

- list_del(&root->link);
+ spin_lock(&kvm->arch.tdp_mmu_pages_lock);
+ list_del_rcu(&root->link);
+ spin_unlock(&kvm->arch.tdp_mmu_pages_lock);

zap_gfn_range(kvm, root, 0, max_gfn, false, false);

- tdp_mmu_free_sp(root);
+ call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
}

/*
@@ -82,18 +100,21 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,

lockdep_assert_held_write(&kvm->mmu_lock);

+ rcu_read_lock();
+
if (prev_root)
- next_root = list_next_entry(prev_root, link);
+ next_root = list_next_or_null_rcu(&kvm->arch.tdp_mmu_roots,
+ &prev_root->link,
+ typeof(*prev_root), link);
else
- next_root = list_first_entry(&kvm->arch.tdp_mmu_roots,
- typeof(*next_root), link);
+ next_root = list_first_or_null_rcu(&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);
+ while (next_root && !kvm_tdp_mmu_get_root(kvm, next_root))
+ next_root = list_next_or_null_rcu(&kvm->arch.tdp_mmu_roots,
+ &next_root->link, typeof(*next_root), link);

- if (list_entry_is_head(next_root, &kvm->arch.tdp_mmu_roots, link))
- next_root = NULL;
+ rcu_read_unlock();

if (prev_root)
kvm_tdp_mmu_put_root(kvm, prev_root);
@@ -107,15 +128,17 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
* if exiting the loop early, the caller must drop the reference to the most
* recent root. (Unless keeping a live reference is desirable.)
*/
-#define for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id) \
+#define for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id) \
for (_root = tdp_mmu_next_root(_kvm, NULL); \
_root; \
_root = tdp_mmu_next_root(_kvm, _root)) \
if (kvm_mmu_page_as_id(_root) != _as_id) { \
} else

-#define for_each_tdp_mmu_root(_kvm, _root, _as_id) \
- list_for_each_entry(_root, &_kvm->arch.tdp_mmu_roots, link) \
+#define for_each_tdp_mmu_root(_kvm, _root, _as_id) \
+ list_for_each_entry_rcu(_root, &_kvm->arch.tdp_mmu_roots, link, \
+ lockdep_is_held_type(&kvm->mmu_lock, 0) || \
+ lockdep_is_help(&kvm->arch.tdp_mmu_pages_lock)) \
if (kvm_mmu_page_as_id(_root) != _as_id) { \
} else

@@ -171,28 +194,14 @@ 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);
refcount_set(&root->tdp_mmu_root_count, 1);

- list_add(&root->link, &kvm->arch.tdp_mmu_roots);
+ spin_lock(&kvm->arch.tdp_mmu_pages_lock);
+ list_add_rcu(&root->link, &kvm->arch.tdp_mmu_roots);
+ spin_unlock(&kvm->arch.tdp_mmu_pages_lock);

out:
return __pa(root->spt);
}

-/*
- * This is called through call_rcu in order to free TDP page table memory
- * safely with respect to other kernel threads that may be operating on
- * the memory.
- * By only accessing TDP MMU page table memory in an RCU read critical
- * section, and freeing it after a grace period, lockless access to that
- * memory won't use it after it is freed.
- */
-static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
-{
- struct kvm_mmu_page *sp = container_of(head, struct kvm_mmu_page,
- rcu_head);
-
- tdp_mmu_free_sp(sp);
-}
-
static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
u64 old_spte, u64 new_spte, int level,
bool shared);
--
2.31.0.208.g409f899ff0-goog