Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary
From: Paolo Bonzini
Date: Wed Mar 31 2021 - 03:53:19 EST
On 26/03/21 03:19, Sean Christopherson wrote:
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().
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.
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>
Please submit this as a separate patch.
Paolo
---
include/linux/kvm_host.h | 8 +-
virt/kvm/kvm_main.c | 174 ++++++++++++++++++++++++++++++---------
2 files changed, 142 insertions(+), 40 deletions(-)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 40ac2d40bb5a..2cc0f87d936e 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -523,6 +523,7 @@ struct kvm {
long mmu_notifier_count;
unsigned long mmu_notifier_range_start;
unsigned long mmu_notifier_range_end;
+ struct rw_semaphore mmu_notifier_slots_lock;
#endif
long tlbs_dirty;
struct list_head devices;
@@ -660,8 +661,11 @@ 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) ||
+#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
+ lockdep_is_held(&kvm->mmu_notifier_slots_lock) ||
+#endif
+ !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 0c2aff8a4aa1..9ebc6d3e4a21 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -453,20 +453,56 @@ static void kvm_mmu_notifier_invalidate_range(struct mmu_notifier *mn,
typedef bool (*hva_handler_t)(struct kvm *kvm, struct kvm_gfn_range *range);
+typedef void (*on_lock_fn_t)(struct kvm *kvm, unsigned long start,
+ unsigned long end);
+
struct kvm_hva_range {
unsigned long start;
unsigned long end;
pte_t pte;
hva_handler_t handler;
- bool caller_locked;
+ on_lock_fn_t on_lock;
+ bool must_lock;
bool flush_on_ret;
bool may_block;
};
+/*
+ * Use a dedicated stub instead of NULL to indicate that there is no callback
+ * function/handler. The compiler technically can't guarantee that a real
+ * function will have a non-zero address, and so it will generate code to
+ * check for !NULL, whereas comparing against a stub will be elided at compile
+ * time (unless the compiler is getting long in the tooth, e.g. gcc 4.9).
+ */
+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)
{
- bool ret = false, locked = range->caller_locked;
+ bool ret = false, locked = false;
struct kvm_gfn_range gfn_range;
struct kvm_memory_slot *slot;
struct kvm_memslots *slots;
@@ -474,6 +510,10 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
idx = srcu_read_lock(&kvm->srcu);
+ 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);
kvm_for_each_memslot(slot, slots) {
@@ -502,10 +542,9 @@ 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);
}
}
@@ -513,7 +552,8 @@ 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);
- if (locked && !range->caller_locked)
+out_unlock:
+ if (locked)
KVM_MMU_UNLOCK(kvm);
srcu_read_unlock(&kvm->srcu, idx);
@@ -534,10 +574,12 @@ static __always_inline int kvm_handle_hva_range(struct mmu_notifier *mn,
.end = end,
.pte = pte,
.handler = handler,
- .caller_locked = false,
+ .on_lock = (void *)kvm_null_fn,
+ .must_lock = false,
.flush_on_ret = true,
.may_block = false,
};
+
return __kvm_handle_hva_range(kvm, &range);
}
@@ -552,7 +594,8 @@ static __always_inline int kvm_handle_hva_range_no_flush(struct mmu_notifier *mn
.end = end,
.pte = __pte(0),
.handler = handler,
- .caller_locked = false,
+ .on_lock = (void *)kvm_null_fn,
+ .must_lock = false,
.flush_on_ret = false,
.may_block = false,
};
@@ -569,23 +612,9 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
}
-static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
- const struct mmu_notifier_range *range)
+static void kvm_inc_notifier_count(struct kvm *kvm, unsigned long start,
+ unsigned long end)
{
- struct kvm *kvm = mmu_notifier_to_kvm(mn);
- const struct kvm_hva_range hva_range = {
- .start = range->start,
- .end = range->end,
- .pte = __pte(0),
- .handler = kvm_unmap_gfn_range,
- .caller_locked = true,
- .flush_on_ret = true,
- .may_block = mmu_notifier_range_blockable(range),
- };
-
- trace_kvm_unmap_hva_range(range->start, range->end);
-
- KVM_MMU_LOCK(kvm);
/*
* The count increase must become visible at unlock time as no
* spte can be established without taking the mmu_lock and
@@ -593,8 +622,8 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
*/
kvm->mmu_notifier_count++;
if (likely(kvm->mmu_notifier_count == 1)) {
- kvm->mmu_notifier_range_start = range->start;
- kvm->mmu_notifier_range_end = range->end;
+ kvm->mmu_notifier_range_start = start;
+ kvm->mmu_notifier_range_end = end;
} else {
/*
* Fully tracking multiple concurrent ranges has dimishing
@@ -606,24 +635,54 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
* complete.
*/
kvm->mmu_notifier_range_start =
- min(kvm->mmu_notifier_range_start, range->start);
+ min(kvm->mmu_notifier_range_start, start);
kvm->mmu_notifier_range_end =
- max(kvm->mmu_notifier_range_end, range->end);
+ max(kvm->mmu_notifier_range_end, end);
}
-
- __kvm_handle_hva_range(kvm, &hva_range);
-
- KVM_MMU_UNLOCK(kvm);
-
- return 0;
}
-static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
+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,
+ .end = range->end,
+ .pte = __pte(0),
+ .handler = kvm_unmap_gfn_range,
+ .on_lock = kvm_inc_notifier_count,
+ .must_lock = !blockable,
+ .flush_on_ret = true,
+ .may_block = blockable,
+ };
- KVM_MMU_LOCK(kvm);
+ 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;
+}
+
+static void kvm_dec_notifier_count(struct kvm *kvm, unsigned long start,
+ unsigned long end)
+{
/*
* This sequence increase will notify the kvm page fault that
* the page that is going to be mapped in the spte could have
@@ -637,7 +696,29 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
* in conjunction with the smp_rmb in mmu_notifier_retry().
*/
kvm->mmu_notifier_count--;
- KVM_MMU_UNLOCK(kvm);
+}
+
+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,
+ .end = range->end,
+ .pte = __pte(0),
+ .handler = (void *)kvm_null_fn,
+ .on_lock = kvm_dec_notifier_count,
+ .must_lock = !blockable,
+ .flush_on_ret = true,
+ .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);
}
@@ -709,6 +790,8 @@ static const struct mmu_notifier_ops kvm_mmu_notifier_ops = {
static int kvm_init_mmu_notifier(struct kvm *kvm)
{
+ init_rwsem(&kvm->mmu_notifier_slots_lock);
+
kvm->mmu_notifier.ops = &kvm_mmu_notifier_ops;
return mmu_notifier_register(&kvm->mmu_notifier, current->mm);
}
@@ -971,6 +1054,15 @@ 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
+ * range_start and range_end. At this point no more MMU notifiers will
+ * run, but the lock could still be held if KVM's notifier was removed
+ * between range_start and range_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
@@ -1222,7 +1314,13 @@ 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;
+#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
+ down_write(&kvm->mmu_notifier_slots_lock);
+#endif
rcu_assign_pointer(kvm->memslots[as_id], slots);
+#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
+ up_write(&kvm->mmu_notifier_slots_lock);
+#endif
synchronize_srcu_expedited(&kvm->srcu);
/*