[PATCH v2 09/13] KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock

Ben Gardon
2021-04-01T23:38:45Z

To reduce lock contention and interference with page fault handlers,
allow the TDP MMU function to zap a GFN range to operate under the MMU
read lock.

Signed-off-by: Ben Gardon <bgardon@google.com>
---
arch/x86/kvm/mmu/mmu.c | 22 +++++---
arch/x86/kvm/mmu/tdp_mmu.c | 111 ++++++++++++++++++++++++++-----------
arch/x86/kvm/mmu/tdp_mmu.h | 14 +++--
3 files changed, 102 insertions(+), 45 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 47d996a8074f..d03a7a8b7ea2 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3154,7 +3154,7 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
sp = to_shadow_page(*root_hpa & PT64_BASE_ADDR_MASK);

if (is_tdp_mmu_page(sp))
- kvm_tdp_mmu_put_root(kvm, sp);
+ kvm_tdp_mmu_put_root(kvm, sp, false);
else if (!--sp->root_count && sp->role.invalid)
kvm_mmu_prepare_zap_page(kvm, sp, invalid_list);

@@ -5507,16 +5507,24 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
}
}

- if (is_tdp_mmu_enabled(kvm)) {
- for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
- flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, gfn_start,
- gfn_end, flush);
- }
-
if (flush)
kvm_flush_remote_tlbs_with_address(kvm, gfn_start, gfn_end);

write_unlock(&kvm->mmu_lock);
+
+ if (is_tdp_mmu_enabled(kvm)) {
+ flush = false;
+
+ read_lock(&kvm->mmu_lock);
+ for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
+ flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, gfn_start,
+ gfn_end, flush, true);
+ if (flush)
+ kvm_flush_remote_tlbs_with_address(kvm, gfn_start,
+ gfn_end);
+
+ read_unlock(&kvm->mmu_lock);
+ }
}

static bool slot_rmap_write_protect(struct kvm *kvm,
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index c1d7f6b86870..6917403484ce 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -27,6 +27,15 @@ void kvm_mmu_init_tdp_mmu(struct kvm *kvm)
INIT_LIST_HEAD(&kvm->arch.tdp_mmu_pages);
}

+static __always_inline void kvm_lockdep_assert_mmu_lock_held(struct kvm *kvm,
+ bool shared)
+{
+ if (shared)
+ lockdep_assert_held_read(&kvm->mmu_lock);
+ else
+ lockdep_assert_held_write(&kvm->mmu_lock);
+}
+
void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
{
if (!kvm->arch.tdp_mmu_enabled)
@@ -42,7 +51,8 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
}

static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
- gfn_t start, gfn_t end, bool can_yield, bool flush);
+ gfn_t start, gfn_t end, bool can_yield, bool flush,
+ bool shared);

static void tdp_mmu_free_sp(struct kvm_mmu_page *sp)
{
@@ -66,11 +76,12 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
tdp_mmu_free_sp(sp);
}

-void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
+void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
+ bool shared)
{
gfn_t max_gfn = 1ULL << (shadow_phys_bits - PAGE_SHIFT);

- lockdep_assert_held_write(&kvm->mmu_lock);
+ kvm_lockdep_assert_mmu_lock_held(kvm, shared);

if (!refcount_dec_and_test(&root->tdp_mmu_root_count))
return;
@@ -81,7 +92,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
list_del_rcu(&root->link);
spin_unlock(&kvm->arch.tdp_mmu_pages_lock);

- zap_gfn_range(kvm, root, 0, max_gfn, false, false);
+ zap_gfn_range(kvm, root, 0, max_gfn, false, false, shared);

call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
}
@@ -94,11 +105,11 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
* function will return NULL.
*/
static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
- struct kvm_mmu_page *prev_root)
+ struct kvm_mmu_page *prev_root,
+ bool shared)
{
struct kvm_mmu_page *next_root;

- lockdep_assert_held_write(&kvm->mmu_lock);

rcu_read_lock();

@@ -117,7 +128,7 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
rcu_read_unlock();

if (prev_root)
- kvm_tdp_mmu_put_root(kvm, prev_root);
+ kvm_tdp_mmu_put_root(kvm, prev_root, shared);

return next_root;
}
@@ -127,12 +138,16 @@ static struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
* This makes it safe to release the MMU lock and yield within the loop, but
* if exiting the loop early, the caller must drop the reference to the most
* recent root. (Unless keeping a live reference is desirable.)
+ *
+ * If shared is set, this function is operating under the MMU lock in read
+ * mode. In the unlikely event that this thread must free a root, the lock
+ * will be temporarily dropped and reacquired in write mode.
*/
-#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) { \
+#define for_each_tdp_mmu_root_yield_safe(_kvm, _root, _as_id, _shared) \
+ for (_root = tdp_mmu_next_root(_kvm, NULL, _shared); \
+ _root; \
+ _root = tdp_mmu_next_root(_kvm, _root, _shared)) \
+ if (kvm_mmu_page_as_id(_root) != _as_id) { \
} else

#define for_each_tdp_mmu_root(_kvm, _root, _as_id) \
@@ -636,7 +651,8 @@ static inline void tdp_mmu_set_spte_no_dirty_log(struct kvm *kvm,
* Return false if a yield was not needed.
*/
static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,
- struct tdp_iter *iter, bool flush)
+ struct tdp_iter *iter, bool flush,
+ bool shared)
{
/* Ensure forward progress has been made before yielding. */
if (iter->next_last_level_gfn == iter->yielded_gfn)
@@ -648,7 +664,11 @@ static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,
if (flush)
kvm_flush_remote_tlbs(kvm);

- cond_resched_rwlock_write(&kvm->mmu_lock);
+ if (shared)
+ cond_resched_rwlock_read(&kvm->mmu_lock);
+ else
+ cond_resched_rwlock_write(&kvm->mmu_lock);
+
rcu_read_lock();

WARN_ON(iter->gfn > iter->next_last_level_gfn);
@@ -666,24 +686,32 @@ static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,
* non-root pages mapping GFNs strictly within that range. Returns true if
* SPTEs have been cleared and a TLB flush is needed before releasing the
* MMU lock.
+ *
* If can_yield is true, will release the MMU lock and reschedule if the
* scheduler needs the CPU or there is contention on the MMU lock. If this
* function cannot yield, it will not release the MMU lock or reschedule and
* the caller must ensure it does not supply too large a GFN range, or the
- * operation can cause a soft lockup. Note, in some use cases a flush may be
- * required by prior actions. Ensure the pending flush is performed prior to
- * yielding.
+ * operation can cause a soft lockup.
+ *
+ * If shared is true, this thread holds the MMU lock in read mode and must
+ * account for the possibility that other threads are modifying the paging
+ * structures concurrently. If shared is false, this thread should hold the
+ * MMU lock in write mode.
*/
static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
- gfn_t start, gfn_t end, bool can_yield, bool flush)
+ gfn_t start, gfn_t end, bool can_yield, bool flush,
+ bool shared)
{
struct tdp_iter iter;

+ kvm_lockdep_assert_mmu_lock_held(kvm, shared);
+
rcu_read_lock();

tdp_root_for_each_pte(iter, root, start, end) {
+retry:
if (can_yield &&
- tdp_mmu_iter_cond_resched(kvm, &iter, flush)) {
+ tdp_mmu_iter_cond_resched(kvm, &iter, flush, shared)) {
flush = false;
continue;
}
@@ -701,8 +729,17 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
!is_last_spte(iter.old_spte, iter.level))
continue;

- tdp_mmu_set_spte(kvm, &iter, 0);
- flush = true;
+ if (!shared) {
+ tdp_mmu_set_spte(kvm, &iter, 0);
+ flush = true;
+ } else if (!tdp_mmu_zap_spte_atomic(kvm, &iter)) {
+ /*
+ * The iter must explicitly re-read the SPTE because
+ * the atomic cmpxchg failed.
+ */
+ iter.old_spte = READ_ONCE(*rcu_dereference(iter.sptep));
+ goto retry;
+ }
}

rcu_read_unlock();
@@ -714,14 +751,21 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
* non-root pages mapping GFNs strictly within that range. Returns true if
* SPTEs have been cleared and a TLB flush is needed before releasing the
* MMU lock.
+ *
+ * If shared is true, this thread holds the MMU lock in read mode and must
+ * account for the possibility that other threads are modifying the paging
+ * structures concurrently. If shared is false, this thread should hold the
+ * MMU in write mode.
*/
bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start,
- gfn_t end, bool can_yield, bool flush)
+ gfn_t end, bool can_yield, bool flush,
+ bool shared)
{
struct kvm_mmu_page *root;

- for_each_tdp_mmu_root_yield_safe(kvm, root, as_id)
- flush = zap_gfn_range(kvm, root, start, end, can_yield, flush);
+ for_each_tdp_mmu_root_yield_safe(kvm, root, as_id, shared)
+ flush = zap_gfn_range(kvm, root, start, end, can_yield, flush,
+ shared);

return flush;
}
@@ -733,7 +777,8 @@ void kvm_tdp_mmu_zap_all(struct kvm *kvm)
int i;

for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
- flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, 0, max_gfn, flush);
+ flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, 0, max_gfn,
+ flush, false);

if (flush)
kvm_flush_remote_tlbs(kvm);
@@ -902,7 +947,7 @@ static __always_inline int kvm_tdp_mmu_handle_hva_range(struct kvm *kvm,
int as_id;

for (as_id = 0; as_id < KVM_ADDRESS_SPACE_NUM; as_id++) {
- for_each_tdp_mmu_root_yield_safe(kvm, root, as_id) {
+ for_each_tdp_mmu_root_yield_safe(kvm, root, as_id, false) {
slots = __kvm_memslots(kvm, as_id);
kvm_for_each_memslot(memslot, slots) {
unsigned long hva_start, hva_end;
@@ -942,7 +987,7 @@ static int zap_gfn_range_hva_wrapper(struct kvm *kvm,
struct kvm_mmu_page *root, gfn_t start,
gfn_t end, unsigned long unused)
{
- return zap_gfn_range(kvm, root, start, end, false, false);
+ return zap_gfn_range(kvm, root, start, end, false, false, false);
}

int kvm_tdp_mmu_zap_hva_range(struct kvm *kvm, unsigned long start,
@@ -1103,7 +1148,7 @@ static bool wrprot_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,

for_each_tdp_pte_min_level(iter, root->spt, root->role.level,
min_level, start, end) {
- if (tdp_mmu_iter_cond_resched(kvm, &iter, false))
+ if (tdp_mmu_iter_cond_resched(kvm, &iter, false, false))
continue;

if (!is_shadow_present_pte(iter.old_spte) ||
@@ -1132,7 +1177,7 @@ bool kvm_tdp_mmu_wrprot_slot(struct kvm *kvm, struct kvm_memory_slot *slot,
struct kvm_mmu_page *root;
bool spte_set = false;

- for_each_tdp_mmu_root_yield_safe(kvm, root, slot->as_id)
+ for_each_tdp_mmu_root_yield_safe(kvm, root, slot->as_id, false)
spte_set |= wrprot_gfn_range(kvm, root, slot->base_gfn,
slot->base_gfn + slot->npages, min_level);

@@ -1156,7 +1201,7 @@ static bool clear_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
rcu_read_lock();

tdp_root_for_each_leaf_pte(iter, root, start, end) {
- if (tdp_mmu_iter_cond_resched(kvm, &iter, false))
+ if (tdp_mmu_iter_cond_resched(kvm, &iter, false, false))
continue;

if (spte_ad_need_write_protect(iter.old_spte)) {
@@ -1191,7 +1236,7 @@ bool kvm_tdp_mmu_clear_dirty_slot(struct kvm *kvm, struct kvm_memory_slot *slot)
struct kvm_mmu_page *root;
bool spte_set = false;

- for_each_tdp_mmu_root_yield_safe(kvm, root, slot->as_id)
+ for_each_tdp_mmu_root_yield_safe(kvm, root, slot->as_id, false)
spte_set |= clear_dirty_gfn_range(kvm, root, slot->base_gfn,
slot->base_gfn + slot->npages);

@@ -1278,7 +1323,7 @@ static bool zap_collapsible_spte_range(struct kvm *kvm,
rcu_read_lock();

tdp_root_for_each_pte(iter, root, start, end) {
- if (tdp_mmu_iter_cond_resched(kvm, &iter, flush)) {
+ if (tdp_mmu_iter_cond_resched(kvm, &iter, flush, false)) {
flush = false;
continue;
}
@@ -1313,7 +1358,7 @@ bool kvm_tdp_mmu_zap_collapsible_sptes(struct kvm *kvm,
{
struct kvm_mmu_page *root;

- for_each_tdp_mmu_root_yield_safe(kvm, root, slot->as_id)
+ for_each_tdp_mmu_root_yield_safe(kvm, root, slot->as_id, false)
flush = zap_collapsible_spte_range(kvm, root, slot, flush);

return flush;
diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
index f0a26214e999..d703c6d6024a 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -13,14 +13,18 @@ __must_check static inline bool kvm_tdp_mmu_get_root(struct kvm *kvm,
return refcount_inc_not_zero(&root->tdp_mmu_root_count);
}

-void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root);
+void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
+ bool shared);

bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start,
- gfn_t end, bool can_yield, bool flush);
+ gfn_t end, bool can_yield, bool flush,
+ bool shared);
static inline bool kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id,
- gfn_t start, gfn_t end, bool flush)
+ gfn_t start, gfn_t end, bool flush,
+ bool shared)
{
- return __kvm_tdp_mmu_zap_gfn_range(kvm, as_id, start, end, true, flush);
+ return __kvm_tdp_mmu_zap_gfn_range(kvm, as_id, start, end, true, flush,
+ shared);
}
static inline bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
{
@@ -37,7 +41,7 @@ static inline bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
*/
lockdep_assert_held_write(&kvm->mmu_lock);
return __kvm_tdp_mmu_zap_gfn_range(kvm, kvm_mmu_page_as_id(sp),
- sp->gfn, end, false, false);
+ sp->gfn, end, false, false, false);
}
void kvm_tdp_mmu_zap_all(struct kvm *kvm);

--
2.31.0.208.g409f899ff0-goog