Re: Redux: Backwards compatibility for XDP multi-buff

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 23/09/2021 15.46, Jakub Kicinski wrote:
> Let me make a general statement that we can't build large systems
> without division of responsibilities. Device specific logic has to
> be left to the driver. It's up to veth to apply its extra rules.
> 
> As Zvi said, tho, this can be left for later. IMHO initial patches can
> drop all mb frames on redirect in the core, so we can make progress.

I agree that for *initial* patchset for REDIRECT (func calling 
ndo_xdp_xmit) it can drop MB frames on redirect in the core, when the 
drivers ndo_xdp_xmit doesn't support this.
BUT only so we can make progress.

As soon as possible (preferably in same kernel release) a follow-up 
patchset should make a serious attempt at updating all drivers 
ndo_xdp_xmit to support multi-buffer (MB).

As MB use the same/compatible layout as skb_shared_info, it should be a 
fairly small effort to update drivers, for *transmitting* multi-buff. As 
drivers already support this for normal SKBs.

It will be a larger effort to support XDP multi-buff on RX, as often the 
RX-loop need to be adjusted/reordered to "delay" calling XDP-prog until 
all fragments have been "collected".  And MTU check adjusted.

This imply that the driver net_device xdp_features need two feature bits 
for this MB feature.
IMHO it makes sense to detangle RX and TX, as IMHO we should allow MB 
frames to be TX redirect out a driver that have an old non-MB aware 
XDP-prog loaded.

--Jesper