Re: [virtio-dev] Re: [PATCH v4] virtio-net: enable configurable tx queue size

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jul 04, 2017 at 07:03:51PM +0800, Wei Wang wrote:
> On 07/04/2017 03:18 AM, Michael S. Tsirkin wrote:
> > On Wed, Jun 28, 2017 at 10:37:59AM +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 crosses a MemoryRegion thereby
> > > generating more than 1024 iovs after translation from guest-physical
> > > address in the backend.
> > > 
> > > Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> > I was going to apply this, but run into a host of issues:
> > 
> > This segfaults:
> > $ ./x86_64-softmmu/qemu-system-x86_64 -device virtio-net,tx_queue_size=1024
> > Segmentation fault (core dumped)
> > 
> > I tried to tweak this code a bit to avoid the crash, and I run into a further issue:
> > $ ./x86_64-softmmu/qemu-system-x86_64 -device virtio-net,tx_queue_size=1024
> > Bad ram offset aa49002
> > Aborted (core dumped)
> > 
> > the second issue is especially concerning.
> > 
> 
> AFAIK, all the virtio-net backends require "-netdev". I'm wondering if there
> is any case that virtio-net can work without a "-netdev" created in QEMU?

Of course. Old style -net with vlans still work.

> If not, would it be better if we just stop the device creation at the
> beginning of
> virtio_net_device_realize() if "-netdev" is not given (i.e.
> !n->nic_conf.peers.ncs[0])?
> 
> Best,
> Wei

That will break a ton of scripts without any real benefit
to users.


-- 
MST

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