On 29/04/21 23:18, Ben Gardon wrote:
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.
Here there should be a blurb about the possible races that can happen
and why we decided for the slots_arch_lock.
Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
---
include/linux/kvm_host.h | 9 +++++++++
virt/kvm/kvm_main.c | 31 ++++++++++++++++++++++++++-----
2 files changed, 35 insertions(+), 5 deletions(-)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 8895b95b6a22..2d5e797fbb08 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -472,6 +472,15 @@ 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 read cirtical section where acquiring the slots_lock
+ * would lead to deadlock with the synchronize_srcu in
+ * install_new_memslots.
+ */
I think usage under slots_lock need not be mentioned here. More like this:
/*
* Protects the arch-specific fields of struct kvm_memory_slots
* in use by the VM. Usually these are initialized by
* kvm_arch_prepare_memory_region and then protected by
* kvm->srcu; however, if they need to be initialized outside
* kvm_arch_prepare_memory_region, slots_arch_lock can
* be used instead as it is also held when calling
* kvm_arch_prepare_memory_region itself. Note that using
* slots_lock would lead to deadlock with install_new_memslots,
* because it is held during synchronize_srcu:
*
* idx = srcu_read_lock(&kvm->srcu);
* mutex_lock(&kvm->slots_lock);
* mutex_lock(&kvm->slots_lock);
* synchronize_srcu(&kvm->srcu);
*/
(Though a better place for this is in
Documentation/virtual/kvm/locking.rst).
Paolo
+ 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];
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index c8010f55e368..97b03fa2d0c8 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -908,6 +908,7 @@ 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);
@@ -1280,6 +1281,10 @@ 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. */
+ mutex_unlock(&kvm->slots_arch_lock);
+
synchronize_srcu_expedited(&kvm->srcu);
/*
@@ -1351,6 +1356,9 @@ static int kvm_set_memslot(struct kvm *kvm,
struct kvm_memslots *slots;
int r;
+ /* Released in install_new_memslots. */
+ mutex_lock(&kvm->slots_arch_lock);
+
slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change);
if (!slots)
return -ENOMEM;
@@ -1364,10 +1372,9 @@ 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);
@@ -1379,6 +1386,17 @@ 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(__kvm_memslots(kvm, as_id), slots);
}
r = kvm_arch_prepare_memory_region(kvm, new, mem, change);
@@ -1394,8 +1412,11 @@ 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);
+ }
kvfree(slots);
return r;
}
