Re: [Qemu-devel] [virtio-dev] Re: [PATCH v3 1/2] virtio-net: enable configurable tx queue size

[Jason Wang]

On 2017年06月26日 12:55, Wei Wang wrote:
> On 06/26/2017 11:18 AM, Jason Wang wrote:
> > On 2017年06月23日 10:32, 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 after translattion from guest-physical
> > > address in the backend.
> > >
> > > Signed-off-by: Wei Wang <address@hidden>
> > > ---
> > >   hw/net/virtio-net.c            | 45 
> > > +++++++++++++++++++++++++++++++++---------
> > >   include/hw/virtio/virtio-net.h |  1 +
> > >   2 files changed, 37 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > > index 91eddaf..d13ca60 100644
> > > --- a/hw/net/virtio-net.c
> > > +++ b/hw/net/virtio-net.c
> > > @@ -34,8 +34,11 @@
> > >     /* previously fixed value */
> > >   #define VIRTIO_NET_RX_QUEUE_DEFAULT_SIZE 256
> > > +
> > > +    /*
> > > +     * 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) {
> > > +        n->net_conf.tx_queue_size = VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE;
> >
> > Do we really want assume all vhost-user backend support 1024 queue size?
>
>  Do you know any vhost-user backend implementation that doesn't support
> 1024 tx queue size?

I don't but the issue is vhost-user uditis an open protocol, there could 
be even closed source implementation on this. So we can't audit all 
kinds of backends.

> > > +    }
> > > +
> > > +    for (i = 0; i < n->max_queues; i++) {
> > > +        virtio_net_add_queue(n, i);
> > > +    }
> >
> > Any reason to move virtio_net_add_queue() here?
>
> Please check the whole init steps.  It was moved here (after 
> qemu_new_nic())
> to make sure nc->peer is not NULL.

I don't get here, can't you get peer just from nic_conf.peers?

Thanks

> Best,
> Wei
>
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