Re: [PATCH 3/3] mlx5_vdpa: defer clear_virtqueues to until DRIVER_OK

From: Si-Wei Liu
Date: Tue Feb 09 2021 - 20:59:19 EST




On 2/8/2021 7:37 PM, Jason Wang wrote:

On 2021/2/6 下午8:29, Si-Wei Liu wrote:
While virtq is stopped,  get_vq_state() is supposed to
be  called to  get  sync'ed  with  the latest internal
avail_index from device. The saved avail_index is used
to restate  the virtq  once device is started.  Commit
b35ccebe3ef7 introduced the clear_virtqueues() routine
to  reset  the saved  avail_index,  however, the index
gets cleared a bit earlier before get_vq_state() tries
to read it. This would cause consistency problems when
virtq is restarted, e.g. through a series of link down
and link up events. We  could  defer  the  clearing of
avail_index  to  until  the  device  is to be started,
i.e. until  VIRTIO_CONFIG_S_DRIVER_OK  is set again in
set_status().

Fixes: b35ccebe3ef7 ("vdpa/mlx5: Restore the hardware used index after change map")
Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
---
  drivers/vdpa/mlx5/net/mlx5_vnet.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
index aa6f8cd..444ab58 100644
--- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
+++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
@@ -1785,7 +1785,6 @@ static void mlx5_vdpa_set_status(struct vdpa_device *vdev, u8 status)
      if (!status) {
          mlx5_vdpa_info(mvdev, "performing device reset\n");
          teardown_driver(ndev);
-        clear_virtqueues(ndev);
          mlx5_vdpa_destroy_mr(&ndev->mvdev);
          ndev->mvdev.status = 0;
          ++mvdev->generation;
@@ -1794,6 +1793,7 @@ static void mlx5_vdpa_set_status(struct vdpa_device *vdev, u8 status)
        if ((status ^ ndev->mvdev.status) & VIRTIO_CONFIG_S_DRIVER_OK) {
          if (status & VIRTIO_CONFIG_S_DRIVER_OK) {
+            clear_virtqueues(ndev);


Rethink about this. As mentioned in another thread, this in fact breaks set_vq_state().  (See vhost_virtqueue_start() -> vhost_vdpa_set_vring_base() in qemu codes).
I assume that the clearing for vhost-vdpa would be done via (qemu code),

vhost_dev_start()->vhost_vdpa_dev_start()->vhost_vdpa_call(status | VIRTIO_CONFIG_S_DRIVER_OK)

which is _after_ vhost_virtqueue_start() gets called to restore the avail_idx to h/w in vhost_dev_start(). What am I missing here?

-Siwei



The issue is that the avail idx is forgot, we need keep it.

Thanks


              err = setup_driver(ndev);
              if (err) {
                  mlx5_vdpa_warn(mvdev, "failed to setup driver\n");