Messages in this thread | | | Subject | Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary | From | Paolo Bonzini <> | Date | Wed, 31 Mar 2021 09:52:24 +0200 |
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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@google.com> > Signed-off-by: Sean Christopherson <seanjc@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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