netdev - Re: [PATCH 3/3] mlx5_vdpa: defer clear_virtqueues to until DRIVER

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Date:   Wed, 10 Feb 2021 12:00:46 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>, mst@...hat.com, elic@...dia.com
Cc:     linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH 3/3] mlx5_vdpa: defer clear_virtqueues to until DRIVER_OK


On 2021/2/10 上午8:26, Si-Wei Liu wrote:
>
>
> 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@...cle.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


I think not. I thought clear_virtqueues() will clear hardware index but 
looks not. (I guess we need a better name other than clear_virtqueues(), 
e.g from the name it looks like the it will clear the hardware states)

Thanks


>
>
>>
>> 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");
>>
>