On 2018/07/25 4:10, Jakub Kicinski wrote:

On Tue, 24 Jul 2018 18:39:09 +0900, Toshiaki Makita wrote:

quoted

On 2018/07/24 10:56, Toshiaki Makita wrote:

quoted

On 2018/07/24 9:27, Jakub Kicinski wrote:  

quoted

On Mon, 23 Jul 2018 00:13:03 +0900, Toshiaki Makita wrote:  

quoted

From: Toshiaki Makita <redacted>

All oversized packets including GSO packets are dropped if XDP is
enabled on receiver side, so don't send such packets from peer.

Drop TSO and SCTP fragmentation features so that veth devices themselves
segment packets with XDP enabled. Also cap MTU accordingly.

Signed-off-by: Toshiaki Makita <redacted>  

Is there any precedence for fixing up features and MTU like this?  Most
drivers just refuse to install the program if settings are incompatible.  

I don't know any precedence. I can refuse the program on installing it
when features and MTU are not appropriate. Is it preferred?
Note that with current implementation wanted_features are not touched so
features will be restored when the XDP program is removed. MTU will not
be restored though, as I do not remember the original MTU.  

I just recalled that virtio_net used to refused XDP when guest offload
features are incompatible but now it dynamically fixup them on
installing an XDP program.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3f93522ffab2d46a36b57adf324a54e674fc9536

That's slightly different AFAIU, because the virtio features weren't
really controllable at runtime at all.  I'm not dead set on leaving the
features be, but I just want to make sure we think this through
properly before we commit to any magic behaviour for ever...

To me it does not look so different. What the above virtio commit is
doing is almost disabling LRO so we could add a feature to toggle LRO
instead but it chose to automatically disable it. And this veth commit
is also almost equivalent to disabling LRO.
IMHO we should do this feature adjustment. It just avoids packet drops
and has no downside. It forces software segmentation on the peer veth.
Features will be restored when XDP is removed so there would be no
surprising for users. It seems there is no benefit to not doing this.

Taking a quick glance at the MTU side, it seems that today if someone
decides to set MTU on one side of a veth pair the packets will simply
get dropped.  So the MTU coupling for XDP doesn't seem in line with
existing behaviour of veth, not only other XDP drivers.

It looks weird to allow such inconsistent MTU settings. But anyway
changing MTU can have negative effect on users, such as causing
fragmentation or EMSGSIZE error on UDP sendmsg() or not restoring MTU. I
think I should not adjust MTU but just cap max_mtu.

-- 
Toshiaki Makita