Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary

[Paolo Bonzini <pbonzini@redhat.com>]

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);
>   
>   	/*
>