netdev - Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support

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Message-ID: <CACAyw98tVmuRbMr5RpPY_0GmU_bQAH+d9=UoEx3u5g+nGSwfYQ@mail.gmail.com>
Date:   Wed, 29 Sep 2021 11:36:33 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, shayagr@...zon.com,
        David Ahern <dsahern@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Saeed Mahameed <saeed@...nel.org>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        tirthendu.sarkar@...el.com
Subject: Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support

On Mon, 20 Sept 2021 at 23:46, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> > The draft API was:
> >
> > void *xdp_mb_pointer_flush(struct xdp_buff *xdp_md, u32 flags,
> >                            u32 offset, u32 len, void *stack_buf)
> >
> > Which does not take the ptr returned by header_pointer(), but that's
> > easy to add (well, easy other than the fact it'd be the 6th arg).
>
> I guess we could play some trickery with stuffing offset/len/flags into
> one or two u64s to save an argument or two?

Adding another pointer arg seems really hard to explain as an API.
What happens if I pass the "wrong" ptr? What happens if I pass NULL?

How about this: instead of taking stack_ptr, take the return value
from header_pointer as an argument. Then use the already existing
(right ;P) inlining to do the following:

   if (md->ptr + args->off != ret_ptr)
     __pointer_flush(...)

This means that __pointer_flush has to deal with aliased memory
though, so it would always have to memmove. Probably OK for the "slow"
path?

Lorenz

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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