Re: [PATCH v2 1/2] virtio-net: enable configurable tx queue size

[Wei Wang <wei.w.wang@intel.com>]

On 06/16/2017 10:31 PM, Michael S. Tsirkin wrote:
> On Fri, Jun 16, 2017 at 06:48:38PM +0800, Wei Wang wrote:
> > This patch enables the virtio-net tx queue size to be configurable
> > between 256 (the default queue size) and 1024 by the user when the
> > vhost-user backend is used.
> >
> > Currently, the maximum tx queue size for other backends is 512 due
> > to the following limitations:
> > - QEMU backend: the QEMU backend implementation in some cases may
> > send 1024+1 iovs to writev.
> > - Vhost_net backend: there are possibilities that the guest sends
> > a vring_desc of memory which corsses a MemoryRegion thereby
> > generating more than 1024 iovs in total after translattion from
> > guest-physical address in the backend.
> >
> > Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> > ---
> >   hw/net/virtio-net.c            | 46 ++++++++++++++++++++++++++++++++++--------
> >   include/hw/virtio/virtio-net.h |  1 +
> >   2 files changed, 39 insertions(+), 8 deletions(-)
> >
> > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > index 7d091c9..e1a08fd 100644
> > --- a/hw/net/virtio-net.c
> > +++ b/hw/net/virtio-net.c
> > @@ -33,8 +33,11 @@
> >   
> >   /* previously fixed value */
> >   #define VIRTIO_NET_RX_QUEUE_DEFAULT_SIZE 256
> > +#define VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE 256
> > +
> >   /* for now, only allow larger queues; with virtio-1, guest can downsize */
> >   #define VIRTIO_NET_RX_QUEUE_MIN_SIZE VIRTIO_NET_RX_QUEUE_DEFAULT_SIZE
> > +#define VIRTIO_NET_TX_QUEUE_MIN_SIZE VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE
> >   
> >   /*
> >    * Calculate the number of bytes up to and including the given 'field' of
> > @@ -1491,18 +1494,33 @@ static void virtio_net_tx_bh(void *opaque)
> >   static void virtio_net_add_queue(VirtIONet *n, int index)
> >   {
> >       VirtIODevice *vdev = VIRTIO_DEVICE(n);
> > +    NetClientState *nc = qemu_get_queue(n->nic);
> >   
> >       n->vqs[index].rx_vq = virtio_add_queue(vdev, n->net_conf.rx_queue_size,
> >                                              virtio_net_handle_rx);
> > +
> > +    /*
> > +     * Currently, backends other than vhost-user don't support 1024 queue
> > +     * size.
> > +     */
> > +    if (n->net_conf.tx_queue_size == VIRTQUEUE_MAX_SIZE &&
> > +        nc->peer->info->type != NET_CLIENT_DRIVER_VHOST_USER) {
> > +        fprintf(stderr, "warning: %s: queue size %d not supported\n",
> > +                __func__, n->net_conf.tx_queue_size);
> > +        n->net_conf.tx_queue_size = VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE;
> > +    }
> > +
> Also, I suspect we can get here with no peer, and above will crash.
> It seems ugly to do this on each virtio_net_add_queue.
> How about moving this to realize?

The code has been re-arranged to make sure nc->peer is ready before
it's used, but I agree that it looks better to move the above to realize().

Best,
Wei