Messages in this thread | | | Date | Sun, 9 Jun 2013 10:11:27 +0300 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH] virtio-net: put virtio net header inline with data |
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On Fri, Jun 07, 2013 at 11:42:43AM +0930, Rusty Russell wrote: > "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...
Nod. Though if we want to require this for all devices, virtio-blk scsi command passthrough will need to change - I sent a spec patch a while ago virtio-spec: add field for scsi command size any comments on it?
> > 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?
I think it's mostly doesn't fail in my testing.
It's probably a good idea to add a counter here, then if it starts triggering we can optimize.
I think things like skb_header_cloned depend on guest config really, e.g. tcpdump running on the interface in guest can cause this.
> Would it suck if we copied > headers which didn't let us prepend data?
I think it will - copies are generally best avoided, and header is easily 1K of data.
> Or could we bump > dev->hard_header_len appropriately?
Needs some thought, though from experience it's a pain.
> Thanks, > Rusty.
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