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Subject[PATCH 2/2] drm/virtio: notify virtqueues without holding spinlock
Date
Split virtqueue_kick() call into virtqueue_kick_prepare(), which
requires serialization, and virtqueue_notify(), which does not. Move
the virtqueue_notify() call out of the critical section protected by the
queue lock. This avoids triggering a vmexit while holding the lock and
thereby fixes a rather bad spinlock contention.

Suggested-by: Chia-I Wu <olvaffe@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
drivers/gpu/drm/virtio/virtgpu_vq.c | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c b/drivers/gpu/drm/virtio/virtgpu_vq.c
index ca91e83ffaef..e41c96143342 100644
--- a/drivers/gpu/drm/virtio/virtgpu_vq.c
+++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
@@ -252,7 +252,7 @@ void virtio_gpu_dequeue_cursor_func(struct work_struct *work)
wake_up(&vgdev->cursorq.ack_queue);
}

-static void virtio_gpu_queue_ctrl_buffer_locked(struct virtio_gpu_device *vgdev,
+static bool virtio_gpu_queue_ctrl_buffer_locked(struct virtio_gpu_device *vgdev,
struct virtio_gpu_vbuffer *vbuf)
__releases(&vgdev->ctrlq.qlock)
__acquires(&vgdev->ctrlq.qlock)
@@ -260,10 +260,11 @@ static void virtio_gpu_queue_ctrl_buffer_locked(struct virtio_gpu_device *vgdev,
struct virtqueue *vq = vgdev->ctrlq.vq;
struct scatterlist *sgs[3], vcmd, vout, vresp;
int outcnt = 0, incnt = 0;
+ bool notify = false;
int ret;

if (!vgdev->vqs_ready)
- return;
+ return notify;

sg_init_one(&vcmd, vbuf->buf, vbuf->size);
sgs[outcnt + incnt] = &vcmd;
@@ -292,16 +293,21 @@ static void 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);
+ notify = virtqueue_kick_prepare(vq);
}
+ return notify;
}

static void virtio_gpu_queue_ctrl_buffer(struct virtio_gpu_device *vgdev,
struct virtio_gpu_vbuffer *vbuf)
{
+ bool notify;
+
spin_lock(&vgdev->ctrlq.qlock);
- virtio_gpu_queue_ctrl_buffer_locked(vgdev, vbuf);
+ notify = virtio_gpu_queue_ctrl_buffer_locked(vgdev, vbuf);
spin_unlock(&vgdev->ctrlq.qlock);
+ if (notify)
+ virtqueue_notify(vgdev->ctrlq.vq);
}

static void virtio_gpu_queue_fenced_ctrl_buffer(struct virtio_gpu_device *vgdev,
@@ -310,6 +316,7 @@ static void virtio_gpu_queue_fenced_ctrl_buffer(struct virtio_gpu_device *vgdev,
struct virtio_gpu_fence *fence)
{
struct virtqueue *vq = vgdev->ctrlq.vq;
+ bool notify;

again:
spin_lock(&vgdev->ctrlq.qlock);
@@ -330,8 +337,10 @@ static void virtio_gpu_queue_fenced_ctrl_buffer(struct virtio_gpu_device *vgdev,

if (fence)
virtio_gpu_fence_emit(vgdev, hdr, fence);
- virtio_gpu_queue_ctrl_buffer_locked(vgdev, vbuf);
+ notify = virtio_gpu_queue_ctrl_buffer_locked(vgdev, vbuf);
spin_unlock(&vgdev->ctrlq.qlock);
+ if (notify)
+ virtqueue_notify(vgdev->ctrlq.vq);
}

static void virtio_gpu_queue_cursor(struct virtio_gpu_device *vgdev,
@@ -339,6 +348,7 @@ static void virtio_gpu_queue_cursor(struct virtio_gpu_device *vgdev,
{
struct virtqueue *vq = vgdev->cursorq.vq;
struct scatterlist *sgs[1], ccmd;
+ bool notify;
int ret;
int outcnt;

@@ -361,10 +371,13 @@ static void 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);
+ notify = virtqueue_kick_prepare(vq);
}

spin_unlock(&vgdev->cursorq.qlock);
+
+ if (notify)
+ virtqueue_notify(vq);
}

/* just create gem objects for userspace and long lived objects,
--
2.18.1
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