[PATCH RFC 06/15] KVM: Introduce dirty ring wait queue

[Peter Xu]

When the dirty ring is completely full, right now we throw an error
message and drop the dirty bit.

A better approach could be that we put the thread onto a waitqueue and
retry after another KVM_RESET_DIRTY_RINGS.

We should still allow the process to be killed, so handle it explicitly.

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 include/linux/kvm_host.h |  1 +
 virt/kvm/kvm_main.c      | 22 ++++++++++++++++------
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 7b747bc9ff3e..a1c9ce5f23a1 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -508,6 +508,7 @@ struct kvm {
 	struct kvm_vm_run *vm_run;
 	u32 dirty_ring_size;
 	struct kvm_dirty_ring vm_dirty_ring;
+	wait_queue_head_t dirty_ring_waitqueue;
 };
 
 #define kvm_err(fmt, ...) \
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 782127d11e9d..bd6172dbff1d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -722,6 +722,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
 	mutex_init(&kvm->irq_lock);
 	mutex_init(&kvm->slots_lock);
 	INIT_LIST_HEAD(&kvm->devices);
+	init_waitqueue_head(&kvm->dirty_ring_waitqueue);
 
 	BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX);
 
@@ -3370,16 +3371,23 @@ static void mark_page_dirty_in_ring(struct kvm *kvm,
 		is_vm_ring = true;
 	}
 
+retry:
 	ret = kvm_dirty_ring_push(ring, indexes,
 				  (as_id << 16)|slot->id, offset,
 				  is_vm_ring);
 	if (ret < 0) {
-		if (is_vm_ring)
-			pr_warn_once("vcpu %d dirty log overflow\n",
-				     vcpu->vcpu_id);
-		else
-			pr_warn_once("per-vm dirty log overflow\n");
-		return;
+		/*
+		 * Ring is full, put us onto per-vm waitqueue and wait
+		 * for another KVM_RESET_DIRTY_RINGS to retry
+		 */
+		wait_event_killable(kvm->dirty_ring_waitqueue,
+				    !kvm_dirty_ring_full(ring));
+
+		/* If we're killed, no worry on lossing dirty bits! */
+		if (fatal_signal_pending(current))
+			return;
+
+		goto retry;
 	}
 
 	if (ret)
@@ -3475,6 +3483,8 @@ static int kvm_vm_ioctl_reset_dirty_pages(struct kvm *kvm)
 	if (cleared)
 		kvm_flush_remote_tlbs(kvm);
 
+	wake_up_all(&kvm->dirty_ring_waitqueue);
+
 	return cleared;
 }
 
-- 
2.21.0