[PATCH] drm/virtio: kick vq outside of the vq lock

[Chia-I Wu]

Replace virtqueue_kick by virtqueue_kick_prepare, which requires
serialization, and virtqueue_notify, which does not.  Repurpose the
return values to indicate whether the vq should be notified.

This fixes a bad spinlock contention when the host is qemu.  When
the guest calls virtqueue_notify, the qemu vcpu thread exits the
guest and waits for the qemu iothread to perform the MMIO.  If the
qemu iothread is still processing the prior buffer, and if the prior
buffer is cheap to GPU, the iothread will go ahead and generate an
IRQ.  A worker thread in the guest might start running
virtio_gpu_dequeue_ctrl_func.  If virtqueue_notify was called with
the vq lock held, the worker thread would have to busy wait inside
virtio_gpu_dequeue_ctrl_func.

v2: fix scrambled commit message

Signed-off-by: Chia-I Wu <olvaffe@xxxxxxxxx>
---
 drivers/gpu/drm/virtio/virtgpu_vq.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c b/drivers/gpu/drm/virtio/virtgpu_vq.c
index 6c1a90717535..e96f88fe5c83 100644
--- a/drivers/gpu/drm/virtio/virtgpu_vq.c
+++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
@@ -291,11 +291,9 @@ static int virtio_gpu_queue_ctrl_buffer_locked(struct virtio_gpu_device *vgdev,
 		trace_virtio_gpu_cmd_queue(vq,
 			(struct virtio_gpu_ctrl_hdr *)vbuf->buf);
 
-		virtqueue_kick(vq);
+		ret = virtqueue_kick_prepare(vq);
 	}
 
-	if (!ret)
-		ret = vq->num_free;
 	return ret;
 }
 
@@ -307,6 +305,10 @@ static int virtio_gpu_queue_ctrl_buffer(struct virtio_gpu_device *vgdev,
 	spin_lock(&vgdev->ctrlq.qlock);
 	rc = virtio_gpu_queue_ctrl_buffer_locked(vgdev, vbuf);
 	spin_unlock(&vgdev->ctrlq.qlock);
+
+	if (rc > 0)
+		virtqueue_notify(vgdev->ctrlq.vq);
+
 	return rc;
 }
 
@@ -339,6 +341,10 @@ static int virtio_gpu_queue_fenced_ctrl_buffer(struct virtio_gpu_device *vgdev,
 		virtio_gpu_fence_emit(vgdev, hdr, fence);
 	rc = virtio_gpu_queue_ctrl_buffer_locked(vgdev, vbuf);
 	spin_unlock(&vgdev->ctrlq.qlock);
+
+	if (rc > 0)
+		virtqueue_notify(vgdev->ctrlq.vq);
+
 	return rc;
 }
 
@@ -369,13 +375,14 @@ static int virtio_gpu_queue_cursor(struct virtio_gpu_device *vgdev,
 		trace_virtio_gpu_cmd_queue(vq,
 			(struct virtio_gpu_ctrl_hdr *)vbuf->buf);
 
-		virtqueue_kick(vq);
+		ret = virtqueue_kick_prepare(vq);
 	}
 
 	spin_unlock(&vgdev->cursorq.qlock);
 
-	if (!ret)
-		ret = vq->num_free;
+	if (ret > 0)
+		virtqueue_notify(vq);
+
 	return ret;
 }
 
-- 
2.22.0.410.gd8fdbe21b5-goog