Re: [Qemu-devel] qemu -netdev tun/tap support can't handle macvtap type devices

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Dec 07, 2016 at 12:01:14PM +0000, Peter Maydell wrote:
> Hi; this is a bug report I'm forwarding for somebody else.
> 
> The problem is that QEMU's tun/tap support in net/tap-linux.c only
> supports the legacy /dev/net/tun interface (the filename is hardcoded).
> If you created the tap device via macvtap, then this is the wrong
> filename, but there's no code in QEMU to get the correct /dev/tapN
> filename from the user-provided ifname=whatever option.
> 
> Talking on IRC, the answer suggested was that we ought to
> do a SIOCGIFINDEX ioctl passing in the user-specified ifname
> string, which then gives you the index N to construct the
> /dev/tapN filename. There is probably complexity in working out
> whether we should do that or use the legacy interface (or try both
> always?) This is probably overall not a very large amount of code,
> though.
> 
> The usual suggested workaround is to use the -netdev fd option, like
>   fd=3 3<>/dev/tap$(< /sys/class/net/tap0/ifindex)
> (which gets the shell to open the right /dev/tap device).
> Unfortunately this isn't compatible with multi-queue support
> because netdev complains
> "ifname=, script=, downscript=, vnet_hdr=, helper=, queues=, fds=,
> and vhostfds= are invalid with fd="
> so you can't pass options like "queues=4"...

FWIW you should be able to instead do

   fds=3 3<>/dev/tap$(< /sys/class/net/tap0/ifindex)

note 'fds' plural, instead of 'fd'

It would be nicer to make it "just work" though when giving a device
name for macvtap

Regards,
Daniel
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