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

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Date:   Mon, 1 Feb 2021 11:17:37 -0800
From:   Si-Wei Liu <siwliu.kernel@...il.com>
To:     Eli Cohen <elic@...dia.com>
Cc:     mst@...hat.com, jasowang@...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 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.
>
> -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
> >