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

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Date:   Tue, 2 Feb 2021 09:06:17 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Eli Cohen <elic@...dia.com>
Cc:     Si-Wei Liu <siwliu.kernel@...il.com>,
        Jason Wang <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 Tue, Feb 02, 2021 at 09:00:55AM +0200, Eli Cohen wrote:
> On Mon, Feb 01, 2021 at 08:15:29PM -0800, 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.
> 
> No relation. That's why I put them in two different patches. Only the
> second one is the fix as I stated in the cover letter.
> 
> Anyway, let's just take the second patch.
> 
> Michael, do you need me to send PATCH 2 again as a single patch or can
> you just take it?

Pls post fixes separately. 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
> > > >>>
> > >