netdev - Re: [PATCH 1/2] vdpa/mlx5: Avoid unnecessary query virtqueue

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Date:   Tue, 2 Feb 2021 14:02:25 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Si-Wei Liu <siwliu.kernel@...il.com>
Cc:     Eli Cohen <elic@...dia.com>, mst@...hat.com,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, lulu@...hat.com,
        Si-Wei Liu <si-wei.liu@...cle.com>
Subject: Re: [PATCH 1/2] vdpa/mlx5: Avoid unnecessary query virtqueue


On 2021/2/2 下午12:15, Si-Wei Liu wrote:
> On Mon, Feb 1, 2021 at 7:13 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>> 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@...il.com> wrote:
>>>> On Thu, Jan 28, 2021 at 5:46 AM Eli Cohen <elic@...dia.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?
> Yep, I think so. To be honest I don't know why it has anything to do
> with the memory hotplug issue.


Eli may know more, my understanding is that, during memory hotplut, qemu 
need to propagate new memory mappings via set_map(). For mellanox, it 
means it needs to rebuild memory keys, so the virtqueue needs to be 
suspended.

Thanks


>
> -Siwei
>
>> Thanks
>>
>>
>>>> -Siwei
>>>>
>>>>
>>>>> Signed-off-by: Eli Cohen <elic@...dia.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
>>>>>