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Subject: Re: [PATCH 1/2] vdpa/mlx5: Avoid unnecessary query virtqueue
To:     Si-Wei Liu <siwliu.kernel@gmail.com>, Eli Cohen <elic@nvidia.com>
Cc:     mst@redhat.com, virtualization@lists.linux-foundation.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        lulu@redhat.com, Si-Wei Liu <si-wei.liu@oracle.com>
References: <20210128134130.3051-1-elic@nvidia.com>
 <20210128134130.3051-2-elic@nvidia.com>
 <CAPWQSg0XtEQ1U5N3a767Ak_naoyPdVF1CeE4r3hmN11a-aoBxg@mail.gmail.com>
 <CAPWQSg3U9DCSK_01Kzuea5B1X+Ef9JB23wBY82A3ss-UXGek_Q@mail.gmail.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <9d6058d6-5ce1-0442-8fd9-5a6fe6a0bc6b@redhat.com>
Date:   Tue, 2 Feb 2021 11:12:51 +0800
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On 2021/2/2 上午3:17, Si-Wei Liu wrote:
> On Mon, Feb 1, 2021 at 10:51 AM Si-Wei Liu <siwliu.kernel@gmail.com> wrote:
>> On Thu, Jan 28, 2021 at 5:46 AM Eli Cohen <elic@nvidia.com> wrote:
>>> suspend_vq should only suspend the VQ on not save the current available
>>> index. This is done when a change of map occurs when the driver calls
>>> save_channel_info().
>> Hmmm, suspend_vq() is also called by teardown_vq(), the latter of
>> which doesn't save the available index as save_channel_info() doesn't
>> get called in that path at all. How does it handle the case that
>> aget_vq_state() is called from userspace (e.g. QEMU) while the
>> hardware VQ object was torn down, but userspace still wants to access
>> the queue index?
>>
>> Refer to https://lore.kernel.org/netdev/1601583511-15138-1-git-send-email-si-wei.liu@oracle.com/
>>
>> vhost VQ 0 ring restore failed: -1: Resource temporarily unavailable (11)
>> vhost VQ 1 ring restore failed: -1: Resource temporarily unavailable (11)
>>
>> QEMU will complain with the above warning while VM is being rebooted
>> or shut down.
>>
>> Looks to me either the kernel driver should cover this requirement, or
>> the userspace has to bear the burden in saving the index and not call
>> into kernel if VQ is destroyed.
> Actually, the userspace doesn't have the insights whether virt queue
> will be destroyed if just changing the device status via set_status().
> Looking at other vdpa driver in tree i.e. ifcvf it doesn't behave like
> so. Hence this still looks to me to be Mellanox specifics and
> mlx5_vdpa implementation detail that shouldn't expose to userspace.


So I think we can simply drop this patch?

Thanks


>> -Siwei
>>
>>
>>> Signed-off-by: Eli Cohen <elic@nvidia.com>
>>> ---
>>>   drivers/vdpa/mlx5/net/mlx5_vnet.c | 8 --------
>>>   1 file changed, 8 deletions(-)
>>>
>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> index 88dde3455bfd..549ded074ff3 100644
>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> @@ -1148,8 +1148,6 @@ static int setup_vq(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtqueue *mvq)
>>>
>>>   static void suspend_vq(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtqueue *mvq)
>>>   {
>>> -       struct mlx5_virtq_attr attr;
>>> -
>>>          if (!mvq->initialized)
>>>                  return;
>>>
>>> @@ -1158,12 +1156,6 @@ static void suspend_vq(struct mlx5_vdpa_net *ndev, struct mlx5_vdpa_virtqueue *m
>>>
>>>          if (modify_virtqueue(ndev, mvq, MLX5_VIRTIO_NET_Q_OBJECT_STATE_SUSPEND))
>>>                  mlx5_vdpa_warn(&ndev->mvdev, "modify to suspend failed\n");
>>> -
>>> -       if (query_virtqueue(ndev, mvq, &attr)) {
>>> -               mlx5_vdpa_warn(&ndev->mvdev, "failed to query virtqueue\n");
>>> -               return;
>>> -       }
>>> -       mvq->avail_idx = attr.available_index;
>>>   }
>>>
>>>   static void suspend_vqs(struct mlx5_vdpa_net *ndev)
>>> --
>>> 2.29.2
>>>