Commit b10a038e authored by Ben Gardon's avatar Ben Gardon Committed by Paolo Bonzini
Browse files

KVM: mmu: Add slots_arch_lock for memslot arch fields 



Add a new lock to protect the arch-specific fields of memslots if they
need to be modified in a kvm->srcu read critical section. A future
commit will use this lock to lazily allocate memslot rmaps for x86.

Signed-off-by: default avatarBen Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-5-bgardon@google.com>
[Add Documentation/ hunk. - Paolo]
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent ddc12f2a

Documentation/virt/kvm/locking.rst

+5 −0
Original line number Diff line number Diff line
@@ -16,6 +16,11 @@ The acquisition orders for mutexes are as follows: 
- kvm->slots_lock is taken outside kvm->irq_lock, though acquiring

  them together is quite rare.



- Unlike kvm->slots_lock, kvm->slots_arch_lock is released before

  synchronize_srcu(&kvm->srcu).  Therefore kvm->slots_arch_lock

  can be taken inside a kvm->srcu read-side critical section,

  while kvm->slots_lock cannot.



On x86:



- vcpu->mutex is taken outside kvm->arch.hyperv.hv_lock

include/linux/kvm_host.h

+9 −0
Original line number Diff line number Diff line
@@ -523,6 +523,15 @@ struct kvm { 
#endif /* KVM_HAVE_MMU_RWLOCK */



	struct mutex slots_lock;



	/*

	 * Protects the arch-specific fields of struct kvm_memory_slots in

	 * use by the VM. To be used under the slots_lock (above) or in a

	 * kvm->srcu critical section where acquiring the slots_lock would

	 * lead to deadlock with the synchronize_srcu in

	 * install_new_memslots.

	 */

	struct mutex slots_arch_lock;

	struct mm_struct *mm; /* userspace tied to this vm */

	struct kvm_memslots __rcu *memslots[KVM_ADDRESS_SPACE_NUM];

	struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];

virt/kvm/kvm_main.c

+48 −6
Original line number Diff line number Diff line
@@ -909,6 +909,7 @@ static struct kvm *kvm_create_vm(unsigned long type) 
	mutex_init(&kvm->lock);

	mutex_init(&kvm->irq_lock);

	mutex_init(&kvm->slots_lock);

	mutex_init(&kvm->slots_arch_lock);

	INIT_LIST_HEAD(&kvm->devices);



	BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX);
@@ -1281,6 +1282,14 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm, 
	slots->generation = gen | KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS;



	rcu_assign_pointer(kvm->memslots[as_id], slots);



	/*

	 * Acquired in kvm_set_memslot. Must be released before synchronize

	 * SRCU below in order to avoid deadlock with another thread

	 * acquiring the slots_arch_lock in an srcu critical section.

	 */

	mutex_unlock(&kvm->slots_arch_lock);



	synchronize_srcu_expedited(&kvm->srcu);



	/*
@@ -1352,9 +1361,27 @@ static int kvm_set_memslot(struct kvm *kvm, 
	struct kvm_memslots *slots;

	int r;



	/*

	 * Released in install_new_memslots.

	 *

	 * Must be held from before the current memslots are copied until

	 * after the new memslots are installed with rcu_assign_pointer,

	 * then released before the synchronize srcu in install_new_memslots.

	 *

	 * When modifying memslots outside of the slots_lock, must be held

	 * before reading the pointer to the current memslots until after all

	 * changes to those memslots are complete.

	 *

	 * These rules ensure that installing new memslots does not lose

	 * changes made to the previous memslots.

	 */

	mutex_lock(&kvm->slots_arch_lock);



	slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change);

	if (!slots)

	if (!slots) {

		mutex_unlock(&kvm->slots_arch_lock);

		return -ENOMEM;

	}



	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {

		/*
@@ -1365,10 +1392,9 @@ static int kvm_set_memslot(struct kvm *kvm, 
		slot->flags |= KVM_MEMSLOT_INVALID;



		/*

		 * We can re-use the old memslots, the only difference from the

		 * newly installed memslots is the invalid flag, which will get

		 * dropped by update_memslots anyway.  We'll also revert to the

		 * old memslots if preparing the new memory region fails.

		 * We can re-use the memory from the old memslots.

		 * It will be overwritten with a copy of the new memslots

		 * after reacquiring the slots_arch_lock below.

		 */

		slots = install_new_memslots(kvm, as_id, slots);
@@ -1380,6 +1406,17 @@ static int kvm_set_memslot(struct kvm *kvm, 
		 *	- kvm_is_visible_gfn (mmu_check_root)

		 */

		kvm_arch_flush_shadow_memslot(kvm, slot);



		/* Released in install_new_memslots. */

		mutex_lock(&kvm->slots_arch_lock);



		/*

		 * The arch-specific fields of the memslots could have changed

		 * between releasing the slots_arch_lock in

		 * install_new_memslots and here, so get a fresh copy of the

		 * slots.

		 */

		kvm_copy_memslots(slots, __kvm_memslots(kvm, as_id));

	}



	r = kvm_arch_prepare_memory_region(kvm, new, mem, change);
@@ -1395,8 +1432,13 @@ static int kvm_set_memslot(struct kvm *kvm, 
	return 0;



out_slots:

	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE)

	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {

		slot = id_to_memslot(slots, old->id);

		slot->flags &= ~KVM_MEMSLOT_INVALID;

		slots = install_new_memslots(kvm, as_id, slots);

	} else {

		mutex_unlock(&kvm->slots_arch_lock);

	}

	kvfree(slots);

	return r;

}