Subject: Re: [PATCH bpf-next RFC 2/3] bpf: devmap dynamic map-value storage area based on BTF

From: Jesper Dangaard Brouer <[email-masked]>
Date: Sat, 30 May 2020 16:36:43 +0200

On Sat, 30 May 2020 00:19:50 -0700
Andrii Nakryiko <[email-masked]> wrote:

> On Fri, May 29, 2020 at 8:59 AM Jesper Dangaard Brouer
> <[email-masked]> wrote:
> >
> > The devmap map-value can be read from BPF-prog side, and could be used for a
> > storage area per device. This could e.g. contain info on headers that need  
> 
> If BPF program needs a storage area per device, why can't it just use
> a separate map or just plain array (both keyed by ifindex) to store
> whatever it needs per-device? It's not clear why this flexibility and
> complexity is needed from the description above.

Sorry I though it was obvious, it is for performance reasons and to
reduce the number of maps needed.  We do a lookup in the devmap anyhow,
thus this memory will be cache-hot.  Doing another lookup in a separate
map, which is not guaranteed to be cache-hot, will be wasting cycles.

> > to be added when packet egress this device.
> >
> > This patchset adds a dynamic storage member to struct bpf_devmap_val. More
> > importantly the struct bpf_devmap_val is made dynamic via leveraging and
> > requiring BTF for struct sizes above 4. The only mandatory struct member is
> > 'ifindex' with a fixed offset of zero.
> >
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > ---
> >  kernel/bpf/devmap.c |  216 ++++++++++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 185 insertions(+), 31 deletions(-)
> >  
> 
> [...]
> 

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer