* [PATCH 1/2] KVM: Block memslot updates across range_start() and range_end()
2021-07-27 17:18 [PATCH v2 0/2] KVM: Don't take mmu_lock for range invalidation unless necessary Paolo Bonzini
@ 2021-07-27 17:18 ` Paolo Bonzini
2021-08-02 18:30 ` Sean Christopherson
2021-07-27 17:18 ` [PATCH 2/2] KVM: Don't take mmu_lock for range invalidation unless necessary Paolo Bonzini
1 sibling, 1 reply; 6+ messages in thread
From: Paolo Bonzini @ 2021-07-27 17:18 UTC (permalink / raw)
To: linux-kernel, kvm; +Cc: seanjc
We would like to avoid taking mmu_lock for .invalidate_range_{start,end}()
notifications that are unrelated to KVM. 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. Therefore, in preparation for this change,
this patch prevents memslot updates across range_start() and range_end().
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.
A long note on the locking: a previous version of the patch used an rwsem
to block the memslot update while the MMU notifier run, but this resulted
in the following deadlock involving the pseudo-lock tagged as
"mmu_notifier_invalidate_range_start".
======================================================
WARNING: possible circular locking dependency detected
5.12.0-rc3+ #6 Tainted: G OE
------------------------------------------------------
qemu-system-x86/3069 is trying to acquire lock:
ffffffff9c775ca0 (mmu_notifier_invalidate_range_start){+.+.}-{0:0}, at: __mmu_notifier_invalidate_range_end+0x5/0x190
but task is already holding lock:
ffffaff7410a9160 (&kvm->mmu_notifier_slots_lock){.+.+}-{3:3}, at: kvm_mmu_notifier_invalidate_range_start+0x36d/0x4f0 [kvm]
which lock already depends on the new lock.
This corresponds to the following MMU notifier logic:
invalidate_range_start
take pseudo lock
down_read() (*)
release pseudo lock
invalidate_range_end
take pseudo lock (**)
up_read()
release pseudo lock
At point (*) we take the mmu_notifiers_slots_lock inside the pseudo lock;
at point (**) we take the pseudo lock inside the mmu_notifiers_slots_lock.
This could cause a deadlock (ignoring for a second that the pseudo lock
is not a lock):
- invalidate_range_start waits on down_read(), because the rwsem is
held by install_new_memslots
- install_new_memslots waits on down_write(), because the rwsem is
held till (another) invalidate_range_end finishes
- invalidate_range_end sits waits on the pseudo lock, held by
invalidate_range_start.
Removing the fairness of the rwsem breaks the cycle (in lockdep terms,
it would change the *shared* rwsem readers into *shared recursive*
readers), so open-code the wait using a readers count and a
spinlock. This also allows handling blockable and non-blockable
critical section in the same way.
Losing the rwsem fairness does theoretically allow MMU notifiers to
block install_new_memslots forever. Note that mm/mmu_notifier.c's own
retry scheme in mmu_interval_read_begin also uses wait/wake_up
and is likewise not fair.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
Documentation/virt/kvm/locking.rst | 6 ++++
include/linux/kvm_host.h | 10 ++++--
virt/kvm/kvm_main.c | 56 ++++++++++++++++++++++++++++--
3 files changed, 67 insertions(+), 5 deletions(-)
diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
index 35eca377543d..8138201efb09 100644
--- a/Documentation/virt/kvm/locking.rst
+++ b/Documentation/virt/kvm/locking.rst
@@ -21,6 +21,12 @@ The acquisition orders for mutexes are as follows:
can be taken inside a kvm->srcu read-side critical section,
while kvm->slots_lock cannot.
+- kvm->mn_active_invalidate_count ensures that pairs of
+ invalidate_range_start() and invalidate_range_end() callbacks
+ use the same memslots array. kvm->slots_lock and kvm->slots_arch_lock
+ are taken on the waiting side in install_new_memslots, so MMU notifiers
+ must not take either kvm->slots_lock or kvm->slots_arch_lock.
+
On x86:
- vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index de58a0890b1a..9d6b4ad407b8 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -548,6 +548,11 @@ struct kvm {
struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM];
struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
+ /* Used to wait for completion of MMU notifiers. */
+ spinlock_t mn_invalidate_lock;
+ unsigned long mn_active_invalidate_count;
+ struct rcuwait mn_memslots_update_rcuwait;
+
/*
* created_vcpus is protected by kvm->lock, and is incremented
* at the beginning of KVM_CREATE_VCPU. online_vcpus is only
@@ -764,8 +769,9 @@ 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) ||
+ READ_ONCE(kvm->mn_active_invalidate_count) ||
+ !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 5cc79373827f..c64a7de60846 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -605,10 +605,8 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
/*
* .change_pte() must be surrounded by .invalidate_range_{start,end}(),
- * and so always runs with an elevated notifier count. This obviates
- * the need to bump the sequence count.
*/
- WARN_ON_ONCE(!kvm->mmu_notifier_count);
+ WARN_ON_ONCE(!READ_ONCE(kvm->mn_active_invalidate_count));
kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
}
@@ -658,6 +656,18 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
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.
+ *
+ * Pairs with the decrement in range_end().
+ */
+ spin_lock(&kvm->mn_invalidate_lock);
+ kvm->mn_active_invalidate_count++;
+ spin_unlock(&kvm->mn_invalidate_lock);
+
__kvm_handle_hva_range(kvm, &hva_range);
return 0;
@@ -694,9 +704,22 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
.flush_on_ret = false,
.may_block = mmu_notifier_range_blockable(range),
};
+ bool wake;
__kvm_handle_hva_range(kvm, &hva_range);
+ /* Pairs with the increment in range_start(). */
+ spin_lock(&kvm->mn_invalidate_lock);
+ wake = (--kvm->mn_active_invalidate_count == 0);
+ spin_unlock(&kvm->mn_invalidate_lock);
+
+ /*
+ * There can only be one waiter, since the wait happens under
+ * slots_lock.
+ */
+ if (wake)
+ rcuwait_wake_up(&kvm->mn_memslots_update_rcuwait);
+
BUG_ON(kvm->mmu_notifier_count < 0);
}
@@ -977,6 +1000,9 @@ static struct kvm *kvm_create_vm(unsigned long type)
mutex_init(&kvm->irq_lock);
mutex_init(&kvm->slots_lock);
mutex_init(&kvm->slots_arch_lock);
+ spin_lock_init(&kvm->mn_invalidate_lock);
+ rcuwait_init(&kvm->mn_memslots_update_rcuwait);
+
INIT_LIST_HEAD(&kvm->devices);
BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX);
@@ -1099,6 +1125,16 @@ 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);
+ /*
+ * At this point, pending calls to invalidate_range_start()
+ * have completed but no more MMU notifiers will run, so
+ * mn_active_invalidate_count may remain unbalanced.
+ * No threads can be waiting in install_new_memslots as the
+ * last reference on KVM has been dropped, but freeing
+ * memslots would deadlock without this manual intervention.
+ */
+ WARN_ON(rcuwait_active(&kvm->mn_memslots_update_rcuwait));
+ kvm->mn_active_invalidate_count = 0;
#else
kvm_arch_flush_shadow_all(kvm);
#endif
@@ -1360,7 +1396,21 @@ 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;
+ /*
+ * Do not store the new memslots while there are invalidations in
+ * progress (preparatory change for the next commit).
+ */
+ spin_lock(&kvm->mn_invalidate_lock);
+ prepare_to_rcuwait(&kvm->mn_memslots_update_rcuwait);
+ while (kvm->mn_active_invalidate_count) {
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ spin_unlock(&kvm->mn_invalidate_lock);
+ schedule();
+ spin_lock(&kvm->mn_invalidate_lock);
+ }
+ finish_rcuwait(&kvm->mn_memslots_update_rcuwait);
rcu_assign_pointer(kvm->memslots[as_id], slots);
+ spin_unlock(&kvm->mn_invalidate_lock);
/*
* Acquired in kvm_set_memslot. Must be released before synchronize
--
2.27.0
^ permalink raw reply related [flat|nested] 6+ messages in thread* [PATCH 2/2] KVM: Don't take mmu_lock for range invalidation unless necessary
2021-07-27 17:18 [PATCH v2 0/2] KVM: Don't take mmu_lock for range invalidation unless necessary Paolo Bonzini
2021-07-27 17:18 ` [PATCH 1/2] KVM: Block memslot updates across range_start() and range_end() Paolo Bonzini
@ 2021-07-27 17:18 ` Paolo Bonzini
2021-08-02 19:05 ` Sean Christopherson
1 sibling, 1 reply; 6+ messages in thread
From: Paolo Bonzini @ 2021-07-27 17:18 UTC (permalink / raw)
To: linux-kernel, kvm; +Cc: seanjc
From: Sean Christopherson <seanjc@google.com>
Avoid taking mmu_lock for .invalidate_range_{start,end}() notifications
that are unrelated to KVM. This is possible now that memslot updates are
blocked from range_start() to range_end(); that ensures that lock elision
happens in both or none, and therefore that mmu_notifier_count updates
(which must occur while holding mmu_lock for write) are always paired
across start->end.
Based on patches originally written by Ben Gardon.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
virt/kvm/kvm_main.c | 24 +++++++++++-------------
1 file changed, 11 insertions(+), 13 deletions(-)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index c64a7de60846..a96cbe24c688 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -496,17 +496,6 @@ 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;
- }
-
for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
slots = __kvm_memslots(kvm, i);
kvm_for_each_memslot(slot, slots) {
@@ -538,6 +527,10 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
if (!locked) {
locked = true;
KVM_MMU_LOCK(kvm);
+ if (!IS_KVM_NULL_FN(range->on_lock))
+ range->on_lock(kvm, range->start, range->end);
+ if (IS_KVM_NULL_FN(range->handler))
+ break;
}
ret |= range->handler(kvm, &gfn_range);
}
@@ -546,7 +539,6 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
if (range->flush_on_ret && (ret || kvm->tlbs_dirty))
kvm_flush_remote_tlbs(kvm);
-out_unlock:
if (locked)
KVM_MMU_UNLOCK(kvm);
@@ -605,8 +597,13 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
/*
* .change_pte() must be surrounded 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.
*/
WARN_ON_ONCE(!READ_ONCE(kvm->mn_active_invalidate_count));
+ if (!kvm->mmu_notifier_count)
+ return;
kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
}
@@ -1398,7 +1395,8 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
/*
* Do not store the new memslots while there are invalidations in
- * progress (preparatory change for the next commit).
+ * progress, otherwise the locking in invalidate_range_start and
+ * invalidate_range_end will be unbalanced.
*/
spin_lock(&kvm->mn_invalidate_lock);
prepare_to_rcuwait(&kvm->mn_memslots_update_rcuwait);
--
2.27.0
^ permalink raw reply related [flat|nested] 6+ messages in thread