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



On Mon, Jan 26, 2009 at 10:39:25AM +0000, Mark McLoughlin wrote:
> IFF_VNET_HDR is a tun/tap flag that allows you to send and receive
> large (i.e. GSO) packets and packets with partial checksums. Setting
> the flag means that every packet is proceeded by the same header which
> virtio uses to communicate GSO/csum metadata.
> 
> 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) QEMU has
> support for this ABI and b) the value of the flag is queryable using
> the TUNGETIFF ioctl.
> 
> It's nearly five months since kvm-74 - the first KVM release with this
> feature - was released. Up until now, we've not added libvirt support
> because there is no clean way to detect support for this in QEMU at
> runtime. A brief attempt to add a "info capabilities" monitor command
> to QEMU floundered. Perfect is the enemy of good enough. Probing the
> KVM version will suffice for now.

ACK to the approach taken here. I've had todo the same version based
check for the migration features, since that has broken compatability
and is also totally undocumented in the command line help output.

Daniel
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