[PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary
Paolo Bonzini
pbonzini at redhat.com
Wed Mar 31 08:52:24 BST 2021
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 at google.com>
> Signed-off-by: Sean Christopherson <seanjc at google.com>
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);
>
> /*
>
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