Re: [PATCH net-next] tun: use xdp_get_frame_len()

[Jesper Dangaard Brouer]

On 08/05/2025 15.29, Willem de Bruijn wrote:
> Jon Kohler wrote:
> > > On May 7, 2025, at 4:56 PM, Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote:
> > >
> > >
> > > Jon Kohler wrote:
> > > > Use xdp_get_frame_len helper to ensure xdp frame size is calculated
> > > > correctly in both single buffer and multi buffer configurations.
> > >
> > > Not necessarily opposed, but multi buffer is not actually possible
> > > in this code path, right?
> > >
> > > tun_put_user_xdp only copies xdp_frame->data, for one.
> > >
> > > Else this would also be fix, not net-next material.
> >
> > Correct, this is a prep patch for future multi buffer support,
> > I’m not aware of any path that can currently do that thru
> > this code.

This is a good example of a performance paper-cut, from my rant.
Adding xdp_get_frame_len() where it is not needed, adds extra code,
in-form of an if-statement and a potential touching of a colder
cache-line in skb_shared_info area.


> > The reason for pursuing multi-buffer is to allow vhost/net
> > batching to work again for large payloads.
>
> I was not aware of that context. I'd add a comment to that in the
> commit message, and send it as part of that series.

It need to part of that series, as that batching change should bring a
larger performance benefit that outweighs the paper-cut.

AFAICR there is also some dual packet handling code path for XDP in
vhost_net/tun.  I'm also willing to take the paper-cut, for cleaning
that up.

--Jesper