On Tue, 8 Dec 2020 11:31:03 +0100
Lorenzo Bianconi <lorenzo.bianconi@xxxxxxxxxx> wrote:
> > On Mon, 2020-12-07 at 22:37 +0100, Maciej Fijalkowski wrote:
> > > On Mon, Dec 07, 2020 at 01:15:00PM -0800, Alexander Duyck wrote:
> > > > On Mon, Dec 7, 2020 at 8:36 AM Lorenzo Bianconi <lorenzo@xxxxxxxxxx
> > > > > wrote:
> > > > > Initialize multi-buffer bit (mb) to 0 in all XDP-capable drivers.
> > > > > This is a preliminary patch to enable xdp multi-buffer support.
> > > > >
> > > > > Signed-off-by: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
> > > >
> > > > I'm really not a fan of this design. Having to update every driver in
> > > > order to initialize a field that was fragmented is a pain. At a
> > > > minimum it seems like it might be time to consider introducing some
> > > > sort of initializer function for this so that you can update things in
> > > > one central place the next time you have to add a new field instead of
> > > > having to update every individual driver that supports XDP. Otherwise
> > > > this isn't going to scale going forward.
+1
> > > Also, a good example of why this might be bothering for us is a fact that
> > > in the meantime the dpaa driver got XDP support and this patch hasn't been
> > > updated to include mb setting in that driver.
> > >
> > something like
> > init_xdp_buff(hard_start, headroom, len, frame_sz, rxq);
> >
> > would work for most of the drivers.
> >
>
> ack, agree. I will add init_xdp_buff() in v6.
I do like the idea of an initialize helper function.
Remember this is fast-path code and likely need to be inlined.
Further more, remember that drivers can and do optimize the number of
writes they do to xdp_buff. There are a number of fields in xdp_buff
that only need to be initialized once per NAPI. E.g. rxq and frame_sz
(some driver do change frame_sz per packet). Thus, you likely need two
inlined helpers for init.
Again, remember that C-compiler will generate an expensive operation
(rep stos) for clearing a struct if it is initialized like this, where
all member are not initialized (do NOT do this):
struct xdp_buff xdp = {
.rxq = rxq,
.frame_sz = PAGE_SIZE,
};
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
