Re: [PATCH 06/13] vhost-user-fs: Use helpers to create/cleanup virtqueue

[Stefan Hajnoczi]

On Mon, Oct 04, 2021 at 03:58:09PM -0400, Vivek Goyal wrote:
> On Mon, Oct 04, 2021 at 02:54:17PM +0100, Stefan Hajnoczi wrote:
> > On Thu, Sep 30, 2021 at 11:30:30AM -0400, Vivek Goyal wrote:
> > > Add helpers to create/cleanup virtuqueues and use those helpers. I will
> > 
> > s/virtuqueues/virtqueues/
> > 
> > > need to reconfigure queues in later patches and using helpers will allow
> > > reusing the code.
> > > 
> > > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > > ---
> > >  hw/virtio/vhost-user-fs.c | 87 +++++++++++++++++++++++----------------
> > >  1 file changed, 52 insertions(+), 35 deletions(-)
> > > 
> > > diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c
> > > index c595957983..d1efbc5b18 100644
> > > --- a/hw/virtio/vhost-user-fs.c
> > > +++ b/hw/virtio/vhost-user-fs.c
> > > @@ -139,6 +139,55 @@ static void vuf_set_status(VirtIODevice *vdev, 
> > > uint8_t status)
> > >      }
> > >  }
> > >  
> > > +static void vuf_handle_output(VirtIODevice *vdev, VirtQueue *vq)
> > > +{
> > > +    /*
> > > +     * Not normally called; it's the daemon that handles the queue;
> > > +     * however virtio's cleanup path can call this.
> > > +     */
> > > +}
> > > +
> > > +static void vuf_create_vqs(VirtIODevice *vdev)
> > > +{
> > > +    VHostUserFS *fs = VHOST_USER_FS(vdev);
> > > +    unsigned int i;
> > > +
> > > +    /* Hiprio queue */
> > > +    fs->hiprio_vq = virtio_add_queue(vdev, fs->conf.queue_size,
> > > +                                     vuf_handle_output);
> > > +
> > > +    /* Request queues */
> > > +    fs->req_vqs = g_new(VirtQueue *, fs->conf.num_request_queues);
> > > +    for (i = 0; i < fs->conf.num_request_queues; i++) {
> > > +        fs->req_vqs[i] = virtio_add_queue(vdev, fs->conf.queue_size,
> > > +                                          vuf_handle_output);
> > > +    }
> > > +
> > > +    /* 1 high prio queue, plus the number configured */
> > > +    fs->vhost_dev.nvqs = 1 + fs->conf.num_request_queues;
> > > +    fs->vhost_dev.vqs = g_new0(struct vhost_virtqueue, 
> > > fs->vhost_dev.nvqs);
> > 
> > These two lines prepare for vhost_dev_init(), so moving them here is
> > debatable. If a caller is going to use this function again in the future
> > then they need to be sure to also call vhost_dev_init(). For now it
> > looks safe, so I guess it's okay.
> 
> Hmm..., I do call this function later from vuf_set_features() and
> reconfigure the queues. I see that I don't call vhost_dev_init()
> in that path. I am not even sure if I should be calling
> vhost_dev_init() from inside vuf_set_features().
> 
> So core reuirement is that at the time of first creating device
> I have no idea if driver supports notification queue or not. So
> I do create device with notification queue. But later if driver
> (and possibly vhost device) does not support notifiation queue,
> then we need to reconfigure queues. What's the correct way to
> do that?

Ah, I see. The simplest approach is to always allocate the maximum
number of virtqueues. QEMU's vhost-user-fs device shouldn't need to
worry about which virtqueues are actually in use. Let virtiofsd (the
vhost-user backend) worry about that.

I posted ideas about how to do that in a reply to another patch in this
series. I can't guarantee it will work, but I think it's worth
exploring.

Stefan

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