On Thu, Oct 04, 2012 at 01:04:56PM +0930, Rusty Russell wrote: > Anthony Liguori <anthony@xxxxxxxxxxxxx> writes: > > Rusty Russell <rusty@xxxxxxxxxxxxxxx> writes: > > > >> "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: > >> > >>> Thinking about Sasha's patches, we can reduce ring usage > >>> for virtio net small packets dramatically if we put > >>> virtio net header inline with the data. > >>> This can be done for free in case guest net stack allocated > >>> extra head room for the packet, and I don't see > >>> why would this have any downsides. > >> > >> I've been wanting to do this for the longest time... but... > >> > >>> Even though with my recent patches qemu > >>> no longer requires header to be the first s/g element, > >>> we need a new feature bit to detect this. > >>> A trivial qemu patch will be sent separately. > >> > >> There's a reason I haven't done this. I really, really dislike "my > >> implemention isn't broken" feature bits. We could have an infinite > >> number of them, for each bug in each device. > > > > This is a bug in the specification. > > > > The QEMU implementation pre-dates the specification. All of the actual > > implementations of virtio relied on the semantics of s/g elements and > > still do. > > lguest fix is pending in my queue. lkvm and qemu are broken; lkvm isn't > ever going to be merged, so I'm not sure what its status is? But I'm > determined to fix qemu, and hence my torture patch to make sure this > doesn't creep in again. If you look at my patch you'll notice there's also a comment in virtio_net.h that seems to be broken in this respect: /* This is the first element of the scatter-gather list. If you don't * specify GSO or CSUM features, you can simply ignore the header. */ There is a similar comment in virtio-blk. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
