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

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Date:   Tue, 9 Feb 2021 16:26:58 -0800
From:   Si-Wei Liu <si-wei.liu@...cle.com>
To:     Jason Wang <jasowang@...hat.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 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


>
> 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");
>