On 26 Jun 2019, at 10:38, Jesper Dangaard Brouer wrote:
On Tue, 25 Jun 2019 03:19:22 +0000
"Machulsky, Zorik" <zo...@amazon.com> wrote:
On 6/23/19, 7:21 AM, "Jesper Dangaard Brouer" <bro...@redhat.com>
wrote:
On Sun, 23 Jun 2019 10:06:49 +0300 <same...@amazon.com> wrote:
> This commit implements the basic functionality of drop/pass
logic in the
> ena driver.
Usually we require a driver to implement all the XDP return
codes,
before we accept it. But as Daniel and I discussed with Zorik
during
NetConf[1], we are going to make an exception and accept the
driver
if you also implement XDP_TX.
As we trust that Zorik/Amazon will follow and implement
XDP_REDIRECT
later, given he/you wants AF_XDP support which requires
XDP_REDIRECT.
Jesper, thanks for your comments and very helpful discussion during
NetConf! That's the plan, as we agreed. From our side I would like to
reiterate again the importance of multi-buffer support by xdp frame.
We would really prefer not to see our MTU shrinking because of xdp
support.
Okay we really need to make a serious attempt to find a way to support
multi-buffer packets with XDP. With the important criteria of not
hurting performance of the single-buffer per packet design.
I've created a design document[2], that I will update based on our
discussions: [2]
https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org
The use-case that really convinced me was Eric's packet header-split.
Lets refresh: Why XDP don't have multi-buffer support:
XDP is designed for maximum performance, which is why certain
driver-level
use-cases were not supported, like multi-buffer packets (like
jumbo-frames).
As it e.g. complicated the driver RX-loop and memory model handling.
The single buffer per packet design, is also tied into eBPF
Direct-Access
(DA) to packet data, which can only be allowed if the packet memory is
in
contiguous memory. This DA feature is essential for XDP performance.
One way forward is to define that XDP only get access to the first
packet buffer, and it cannot see subsequent buffers. For XDP_TX and
XDP_REDIRECT to work then XDP still need to carry pointers (plus
len+offset) to the other buffers, which is 16 bytes per extra buffer.
I’ve seen various network processor HW designs, and they normally get
the first x bytes (128 - 512) which they can manipulate
(append/prepend/insert/modify/delete).
There are designs where they can “page in” the additional fragments
but it’s expensive as it requires additional memory transfers. But the
majority do not care (cannot change) the remaining fragments. Can also
not think of a reason why you might want to remove something at the end
of the frame (thinking about routing/forwarding needs here).
If we do want XDP to access other fragments we could do this through a
helper which swaps the packet context?
//Eelco
