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

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Date:   Tue, 2 Feb 2021 09:06:31 +0200
From:   Eli Cohen <elic@...dia.com>
To:     Jason Wang <jasowang@...hat.com>
CC:     Si-Wei Liu <siwliu.kernel@...il.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 Tue, Feb 02, 2021 at 02:02:25PM +0800, Jason Wang wrote:
> 
> 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.
> 

I think Siwei was asking why the first patch was related to the hotplug
issue.

But you're correct. When memory is added, I get a new memory map. This
requires me to build a new memory key object which covers the new memory
map. Since the virtqueue objects are referencing this memory key, I need
to destroy them and build new ones referncing the new memory key.

> 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
> > > > > > 
>