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authorBen Gardon <bgardon@google.com>2021-05-18 10:34:11 -0700
committerPaolo Bonzini <pbonzini@redhat.com>2021-06-17 13:09:26 -0400
commitb10a038e84d188e15819058b2978b2daa9853aeb (patch)
tree6a3215a5a0ee02fcfd0e3da7fb1456d1a4756274 /virt
parentddc12f2a12917c10b0deb0928f0560bffb7729ec (diff)
downloadlwn-b10a038e84d188e15819058b2978b2daa9853aeb.tar.gz
lwn-b10a038e84d188e15819058b2978b2daa9853aeb.zip
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>
Diffstat (limited to 'virt')
-rw-r--r--virt/kvm/kvm_main.c54
1 files changed, 48 insertions, 6 deletions
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d65be9461493..fa7e7ebefc79 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;
}