QEMU tap-bsd: use user-specified tap device if it already exists

[emaste]

Summary

Previously we would try cloning /dev/tap and then setting the name of the cloned result to the provided tap ifname. Instead, just use the provided ifname if it exists.

[emaste]

hopefully I haven't confused you too much by uploading a QEMU diff to our phabricator :)

[jhb]

net/tap-bsd.c

142

Do we even support this on a cloned tap device? I think it gets whatever name is assigned when it is created and can never be changed. I guess it's fine to let the ioctl fail, but it might be clearer to just strcmp the name with the name from TAPGIFNAME and fail if they don't match?

[emaste]

net/tap-bsd.c

142

Hmm, interesting - I guess this changes the interface name but not the /dev/tap name. I'm not sure what the intended use is, but this is @royger's recent change so we should be able to find that out.

[royger]

LGTM. As spoken on IRC with emaste, we could also strcmp ifname and if it starts with "tap" assume it's already created.

net/tap-bsd.c

142

Yes, only the interface name is changed, the device name stays the same. This is important for Xen because the Xen toolstack expects a specific name for each guest/interface in order to add them to the bridge specified in the config file.

[royger]

Committed as:

http://git.qemu.org/?p=qemu.git;a=commit;h=bd54a9f9435c85de190a82019faef16c5ecf8e46