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

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.