Re: [PATCH] kvm: qemu: Handle tap fds with IFF_VNET_HDR

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

On Wed, Aug 20, 2008 at 12:49:23PM -0500, Anthony Liguori wrote:
> Mark McLoughlin wrote:
> >Allow users to pass an IFF_VNET_HDR tap fd via "-net tap,fd=X"
> >by querying the fd with the recently added TUNGETIFF ioctl() to
> >see if IFF_VNET_HDR has been enabled.
> >
> >Note: users wishing to pass an IFF_VNET_HDR tap fd to qemu
> >should check both that a) this version of qemu can handle
> >such an fd and b) that the TUNGETIFF ioctl() is available in
> >the running kernel.
> >
> >We add a comment to the "qemu -help" output to indicate to
> >users that this version of qemu supports IFF_VNET_HDR.
> >  
> 
> The -help output is not a supported interface.  An info command in the 
> monitor would be a better way to detect this.

The reason mark suggested -help is that libvirt already looks at -help
output to determine what command line args are supported. Whether its
stable or not, its the only way to determine supported args. Hopefully
in the future we can just look for presence of -config, and then just
use a config file to launch QEMU, so we'll have minimal dependance on
this data 'format' for -help :-)

The IFF_VNET_HDR stuff is a little different - its more a capability
of the virtualization engine that a startup option. I'd welcome some
kind of way to query capabilities in general, such as the -capabilites
arg suggested. This would include stuff like the list of supported
NIC models, supported disk backends, supported disk formats, sound
hardware, etc.

Daniel
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