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Date:   Mon, 11 Mar 2019 02:35:37 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Pankaj Gupta <pagupta@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, amit@kernel.org,
        arnd@arndb.de, gregkh@linuxfoundation.org, xiaohli@redhat.com,
        Gal Hammer <ghammer@redhat.com>
Subject: Re: [PATCH] virtio_console: free unused buffers with virtio port
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On Tue, Mar 05, 2019 at 02:09:06AM -0500, Pankaj Gupta wrote:
> 
> Hello Michael,
> 
> Thanks for your reply.
> 
> > 
> > On Mon, Mar 04, 2019 at 06:35:11PM +0530, Pankaj Gupta wrote:
> > >   The commit a7a69ec0d8e4 ("virtio_console: free buffers after reset")
> > >   deffered detaching of unused buffer to virtio device unplug time.
> > > 
> > >   This causes unplug/replug of single port in virtio device with an
> > >   error "Error allocating inbufs\n". As we don't free the unused buffers
> > >   attached with the port. Re-plug the same port tries to allocate new
> > >   buffers in virtqueue and results in this error if queue is full.

That's the basic issue, isn't it? Why aren't we
reusing buffers that are already there?


> > > 
> > >   This patch removes the unused buffers in vq's when we unplug the port.
> > >   This is the best we can do as we cannot call device_reset because virtio
> > >   device is still active. This was the working behaviour before the change
> > >   introduced in commit b3258ff1d6.
> > > 
> > >   Reported-by: Xiaohui Li <xiaohli@redhat.com>
> > >   Fixes: b3258ff1d6 ("virtio_console: free buffers after reset")
> > >   Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
> > 
> > I think if you do this you need to add support
> > in the packed ring.
> 
> o.k. I will look at the implementation details for "support
> of packed ring" for virtio_console. This will take some time.
> 
> Meanwhile "virtio_console" port hotplug/unplug is broken in upstream.
> Can we accept this patch as it fixes the upstream and together
> with parent patch(b3258ff1d6) does nice cleanups as well.
> 
> Thanks,
> Pankaj  

Sorry, no - I don't think we should fix one configuration by breaking the other.
If you want to go back, then that's a spec violation, but I guess we can
fix the spec to match.  OK, but code-wise if you call
virtqueue_detach_unused_buf without device reset then you need to teach
packed ring code to support that.


> > 
> > > ---
> > >  drivers/char/virtio_console.c | 14 +++++++++++---
> > >  1 file changed, 11 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> > > index fbeb71953526..5fbf2ac73111 100644
> > > --- a/drivers/char/virtio_console.c
> > > +++ b/drivers/char/virtio_console.c
> > > @@ -1506,15 +1506,25 @@ static void remove_port(struct kref *kref)
> > >          kfree(port);
> > >  }
> > >  
> > > +static void remove_unused_bufs(struct virtqueue *vq)
> > > +{
> > > +        struct port_buffer *buf;
> > > +
> > > +        while ((buf = virtqueue_detach_unused_buf(vq)))
> > > +                free_buf(buf, true);
> > > +}
> > > +
> > >  static void remove_port_data(struct port *port)
> > >  {
> > >          spin_lock_irq(&port->inbuf_lock);
> > >          /* Remove unused data this port might have received. */
> > >          discard_port_data(port);
> > > +        remove_unused_bufs(port->in_vq);
> > >          spin_unlock_irq(&port->inbuf_lock);
> > >  
> > >          spin_lock_irq(&port->outvq_lock);
> > >          reclaim_consumed_buffers(port);
> > > +        remove_unused_bufs(port->out_vq);
> > >          spin_unlock_irq(&port->outvq_lock);
> > >  }
> > >  
> > > @@ -1950,11 +1960,9 @@ static void remove_vqs(struct ports_device *portdev)
> > >          struct virtqueue *vq;
> > >  
> > >          virtio_device_for_each_vq(portdev->vdev, vq) {
> > > -                struct port_buffer *buf;
> > >  
> > >                  flush_bufs(vq, true);
> > > -                while ((buf = virtqueue_detach_unused_buf(vq)))
> > > -                        free_buf(buf, true);
> > > +                remove_unused_bufs(vq);
> > >          }
> > >          portdev->vdev->config->del_vqs(portdev->vdev);
> > >          kfree(portdev->in_vqs);
> > > --
> > > 2.20.1
> >