Avoid taking mmu_lock for unrelated .invalidate_range_{start,end}()
notifications. Because mmu_notifier_count must be modified while holding
mmu_lock for write, and must always be paired across start->end to stay
balanced, lock elision must happen in both or none. To meet that
requirement, add a rwsem to prevent memslot updates across range_start()
and range_end().
Use a rwsem instead of a rwlock since most notifiers _allow_ blocking,
and the lock will be endl across the entire start() ... end() sequence.
If anything in the sequence sleeps, including the caller or a different
notifier, holding the spinlock would be disastrous.
For notifiers that _disallow_ blocking, e.g. OOM reaping, simply go down
the slow path of unconditionally acquiring mmu_lock. The sane
alternative would be to try to acquire the lock and force the notifier
to retry on failure. But since OOM is currently the _only_ scenario
where blocking is disallowed attempting to optimize a guest that has been
marked for death is pointless.
Unconditionally define and use mmu_notifier_slots_lock in the memslots
code, purely to avoid more #ifdefs. The overhead of acquiring the lock
is negligible when the lock is uncontested, which will always be the case
when the MMU notifiers are not used.
Note, technically flag-only memslot updates could be allowed in parallel,
but stalling a memslot update for a relatively short amount of time is
not a scalability issue, and this is all more than complex enough.
Based heavily on code from Ben Gardon.
Suggested-by: Ben Gardon <bgardon@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
include/linux/kvm_host.h | 6 ++-
virt/kvm/kvm_main.c | 96 +++++++++++++++++++++++++++++++---------
2 files changed, 80 insertions(+), 22 deletions(-)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 40ac2d40bb5a..bc3dd2838bb8 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -472,6 +472,7 @@ struct kvm {
#endif /* KVM_HAVE_MMU_RWLOCK */
struct mutex slots_lock;
+ struct rw_semaphore mmu_notifier_slots_lock;
struct mm_struct *mm; /* userspace tied to this vm */
struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM];
struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
@@ -660,8 +661,9 @@ static inline struct kvm_memslots *__kvm_memslots(struct kvm *kvm, int as_id)
{
as_id = array_index_nospec(as_id, KVM_ADDRESS_SPACE_NUM);
return srcu_dereference_check(kvm->memslots[as_id], &kvm->srcu,
- lockdep_is_held(&kvm->slots_lock) ||
- !refcount_read(&kvm->users_count));
+ lockdep_is_held(&kvm->slots_lock) ||
+ lockdep_is_held(&kvm->mmu_notifier_slots_lock) ||
+ !refcount_read(&kvm->users_count));
}
static inline struct kvm_memslots *kvm_memslots(struct kvm *kvm)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index f6697ad741ed..af28f39817a5 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -462,6 +462,7 @@ struct kvm_hva_range {
pte_t pte;
hva_handler_t handler;
on_lock_fn_t on_lock;
+ bool must_lock;
bool flush_on_ret;
bool may_block;
};
@@ -479,6 +480,25 @@ static void kvm_null_fn(void)
}
#define IS_KVM_NULL_FN(fn) ((fn) == (void *)kvm_null_fn)
+
+/* Acquire mmu_lock if necessary. Returns %true if @handler is "null" */
+static __always_inline bool kvm_mmu_lock_and_check_handler(struct kvm *kvm,
+ const struct kvm_hva_range *range,
+ bool *locked)
+{
+ if (*locked)
+ return false;
+
+ *locked = true;
+
+ KVM_MMU_LOCK(kvm);
+
+ if (!IS_KVM_NULL_FN(range->on_lock))
+ range->on_lock(kvm, range->start, range->end);
+
+ return IS_KVM_NULL_FN(range->handler);
+}
+
static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
const struct kvm_hva_range *range)
{
@@ -495,16 +515,9 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
idx = srcu_read_lock(&kvm->srcu);
- /* The on_lock() path does not yet support lock elision. */
- if (!IS_KVM_NULL_FN(range->on_lock)) {
- locked = true;
- KVM_MMU_LOCK(kvm);
-
- range->on_lock(kvm, range->start, range->end);
-
- if (IS_KVM_NULL_FN(range->handler))
- goto out_unlock;
- }
+ if (range->must_lock &&
+ kvm_mmu_lock_and_check_handler(kvm, range, &locked))
+ goto out_unlock;
for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
slots = __kvm_memslots(kvm, i);
@@ -534,10 +547,9 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
gfn_range.end = hva_to_gfn_memslot(hva_end + PAGE_SIZE - 1, slot);
gfn_range.slot = slot;
- if (!locked) {
- locked = true;
- KVM_MMU_LOCK(kvm);
- }
+ if (kvm_mmu_lock_and_check_handler(kvm, range, &locked))
+ goto out_unlock;
+
ret |= range->handler(kvm, &gfn_range);
}
}
@@ -568,6 +580,7 @@ static __always_inline int kvm_handle_hva_range(struct mmu_notifier *mn,
.pte = pte,
.handler = handler,
.on_lock = (void *)kvm_null_fn,
+ .must_lock = false,
.flush_on_ret = true,
.may_block = false,
};
@@ -587,6 +600,7 @@ static __always_inline int kvm_handle_hva_range_no_flush(struct mmu_notifier *mn
.pte = __pte(0),
.handler = handler,
.on_lock = (void *)kvm_null_fn,
+ .must_lock = false,
.flush_on_ret = false,
.may_block = false,
};
@@ -603,11 +617,15 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
trace_kvm_set_spte_hva(address);
/*
- * .change_pte() must be bookended by .invalidate_range_{start,end}(),
- * and so always runs with an elevated notifier count. This obviates
- * the need to bump the sequence count.
+ * .change_pte() must be bookended by .invalidate_range_{start,end}().
+ * If mmu_notifier_count is zero, then start() didn't find a relevant
+ * memslot and wasn't forced down the slow path; rechecking here is
+ * unnecessary. This can only occur if memslot updates are blocked.
*/
- WARN_ON_ONCE(!kvm->mmu_notifier_count);
+ if (!kvm->mmu_notifier_count) {
+ lockdep_assert_held(&kvm->mmu_notifier_slots_lock);
+ return;
+ }
kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
}
@@ -644,6 +662,7 @@ static void kvm_inc_notifier_count(struct kvm *kvm, unsigned long start,
static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
const struct mmu_notifier_range *range)
{
+ bool blockable = mmu_notifier_range_blockable(range);
struct kvm *kvm = mmu_notifier_to_kvm(mn);
const struct kvm_hva_range hva_range = {
.start = range->start,
@@ -651,12 +670,29 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
.pte = __pte(0),
.handler = kvm_unmap_gfn_range,
.on_lock = kvm_inc_notifier_count,
+ .must_lock = !blockable,
.flush_on_ret = true,
- .may_block = mmu_notifier_range_blockable(range),
+ .may_block = blockable,
};
trace_kvm_unmap_hva_range(range->start, range->end);
+ /*
+ * Prevent memslot modification between range_start() and range_end()
+ * so that conditionally locking provides the same result in both
+ * functions. Without that guarantee, the mmu_notifier_count
+ * adjustments will be imbalanced.
+ *
+ * Skip the memslot-lookup lock elision (set @must_lock above) to avoid
+ * having to take the semaphore on non-blockable calls, e.g. OOM kill.
+ * The complexity required to handle conditional locking for this case
+ * is not worth the marginal benefits, the VM is likely doomed anyways.
+ *
+ * Pairs with the unlock in range_end().
+ */
+ if (blockable)
+ down_read(&kvm->mmu_notifier_slots_lock);
+
__kvm_handle_hva_range(kvm, &hva_range);
return 0;
@@ -683,6 +719,7 @@ static void kvm_dec_notifier_count(struct kvm *kvm, unsigned long start,
static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
const struct mmu_notifier_range *range)
{
+ bool blockable = mmu_notifier_range_blockable(range);
struct kvm *kvm = mmu_notifier_to_kvm(mn);
const struct kvm_hva_range hva_range = {
.start = range->start,
@@ -690,12 +727,17 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
.pte = __pte(0),
.handler = (void *)kvm_null_fn,
.on_lock = kvm_dec_notifier_count,
+ .must_lock = !blockable,
.flush_on_ret = true,
- .may_block = mmu_notifier_range_blockable(range),
+ .may_block = blockable,
};
__kvm_handle_hva_range(kvm, &hva_range);
+ /* Pairs with the lock in range_start(). */
+ if (blockable)
+ up_read(&kvm->mmu_notifier_slots_lock);
+
BUG_ON(kvm->mmu_notifier_count < 0);
}
@@ -908,6 +950,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
mutex_init(&kvm->lock);
mutex_init(&kvm->irq_lock);
mutex_init(&kvm->slots_lock);
+ init_rwsem(&kvm->mmu_notifier_slots_lock);
INIT_LIST_HEAD(&kvm->devices);
BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX);
@@ -1028,6 +1071,16 @@ static void kvm_destroy_vm(struct kvm *kvm)
kvm_coalesced_mmio_free(kvm);
#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
mmu_notifier_unregister(&kvm->mmu_notifier, kvm->mm);
+ /*
+ * Reset the lock used to prevent memslot updates between MMU notifier
+ * invalidate_range_start() and invalidate_range_end(). At this point,
+ * no more MMU notifiers will run and pending calls to ...start() have
+ * completed. But, the lock could still be held if KVM's notifier was
+ * removed between ...start() and ...end(). No threads can be waiting
+ * on the lock as the last reference on KVM has been dropped. If the
+ * lock is still held, freeing memslots will deadlock.
+ */
+ init_rwsem(&kvm->mmu_notifier_slots_lock);
#else
kvm_arch_flush_shadow_all(kvm);
#endif
@@ -1279,7 +1332,10 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
WARN_ON(gen & KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS);
slots->generation = gen | KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS;
+ down_write(&kvm->mmu_notifier_slots_lock);
rcu_assign_pointer(kvm->memslots[as_id], slots);
+ up_write(&kvm->mmu_notifier_slots_lock);
+
synchronize_srcu_expedited(&kvm->srcu);
/*
--
2.31.0.208.g409f899ff0-goog
