On 16-11-03 05:34 PM, Michael S. Tsirkin wrote:

On Thu, Nov 03, 2016 at 04:29:22PM -0700, John Fastabend wrote:

quoted

[...]

quoted

quoted

quoted

- when XDP is attached disable all LRO using VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET
  (not used by driver so far, designed to allow dynamic LRO control with
   ethtool)

I see there is a UAPI bit for this but I guess we also need to add
support to vhost as well? Seems otherwise we may just drop a bunch
of packets on the floor out of handle_rx() when recvmsg returns larger
than a page size. Or did I read this wrong...

It's already supported host side. However you might
get some packets that were in flight when you attached.

Really I must have missed it I don't see any *GUEST_FEATURES* flag in
./drivers/vhost/?

It's all done by QEMU catching these commands and calling
ioctls on the tun/macvtap/packet socket.

Well at least for the tap vhost backend in linux that I found here,

 ./qemu/net/tap-linux.c

there is no LRO feature flag but that is OK I can get it working next
week looks fairly straight forward.

[...]

quoted

And if I try to merge the last email I sent out here. In mergeable and
big_packets modes if LRO is off and MTU < PAGE_SIZE it seems we should
always get physically contiguous data on a single page correct?

Unfortunately not in the mergeable buffer case according to spec, even though
linux hosts will do that, so it's fine to optimize for that
but need to somehow work in other cases e.g. by doing a data copy.

ah OK this makes sense I was looking at vhost implementation in Linux.

quoted

It
may be at some offset in a page however. But the offset should not
matter to XDP. If I read this right we wouldn't need to add a new
XDP mode and could just use the existing merge or big modes. This would
seem cleaner to me than adding a new mode and requiring a qemu option.

Thanks for all the pointers by the way its very helpful.

So for mergeable we spend cycles trying to make buffers as small
as possible and I have a patch to avoid copies for that too,
I'll post it next week hopefully.

Good to know. I'll get the XDP stuff wrapped up next week or see
if Srijeet wants to do it.

Thanks,
John