Virtualization - Feb 2021 - [PATCH 3/3] mlx5_vdpa: defer clear_virtqueues to until DRIVER_OK

On 2021/2/6 ??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 at oracle.com>
> ---
>   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).

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");

On 2/8/2021 7:37 PM, Jason Wang wrote:
>
> On 2021/2/6 ??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 at oracle.com>
>> ---
>> ? 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");
>