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Re: [libvirt] [PATCH] kvm/virtio: Set IFF_VNET_HDR when setting up tap fds



On Wed, 2008-08-13 at 16:01 +0100, Daniel P. Berrange wrote:
> On Wed, Aug 13, 2008 at 03:53:27PM +0100, Mark McLoughlin wrote:
> > By enabling this flag on the tap fds we create, we greatly increase
> > the achievable throughput with virtio_net.
> > 
> > However, we need to be careful to only set the flag when a) KVM has
> > support for this ABI and b) the value of the flag is queryable using
> > the TUNGETIFF ioctl.
> 
> This suggests to me that QEMU should be a the one enabling this
> flag, not libvirt.

It has to be done before adding it to the bridge and can't be changed
afterwards.

(Which perhaps suggests the kernel API is a little dumb, but ...)

>  Its going to need todo this anyway for the
> scenario where QEMU allocates the TAP fd, so it might as well
> enable it for pre-allocated TAP fds passed in from external
> apps.

Yeah, it already does it for the fds it allocates.

Cheers,
Mark.


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