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

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

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?

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