Messages in this thread | | | From | Rusty Russell <> | Subject | Re: [PATCH] virtio-net: put virtio net header inline with data | Date | Fri, 07 Jun 2013 11:42:43 +0930 |
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"Michael S. Tsirkin" <mst@redhat.com> writes: > For small packets we can simplify xmit processing by linearizing buffers > with the header: most packets seem to have enough head room we can use > for this purpose. > > Since some older hypervisors (e.g. qemu before version 1.5) > required that header is the first s/g element, > we need a feature bit for this.
OK, we know this is horrible. But I will sleep better knowing that we this feature need never make it into a final 1.0 spec, since it can be assumed at that point...
> pr_debug("%s: xmit %p %pM\n", vi->dev->name, skb, dest); > + if (vi->mergeable_rx_bufs) > + hdr_len = sizeof hdr->mhdr; > + else > + hdr_len = sizeof hdr->hdr; > + > + can_push = vi->any_header_sg && > + !((unsigned long)skb->data & (__alignof__(*hdr) - 1)) && > + !skb_header_cloned(skb) && skb_headroom(skb) >= hdr_len;
Idle thought: how often does this fail? Would it suck if we copied headers which didn't let us prepend data? Or could we bump dev->hard_header_len appropriately?
Thanks, Rusty.
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