From b10a038e84d188e15819058b2978b2daa9853aeb Mon Sep 17 00:00:00 2001
From: Ben Gardon <bgardon@google.com>
Date: Tue, 18 May 2021 10:34:11 -0700
Subject: [PATCH] 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: Ben Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-5-bgardon@google.com>
[Add Documentation/ hunk. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
Documentation/virt/kvm/locking.rst | 5 +++
include/linux/kvm_host.h | 9 +++++
virt/kvm/kvm_main.c | 54 ++++++++++++++++++++++++++----
3 files changed, 62 insertions(+), 6 deletions(-)
diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
index 1fc860c007a3c..35eca377543df 100644
--- a/Documentation/virt/kvm/locking.rst
+++ b/Documentation/virt/kvm/locking.rst
@@ -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
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 8583ed3ff3447..11b9b11a5e9b0 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -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];
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d65be94614937..fa7e7ebefc796 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -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;
}