netdev - Re: [PATCH bpf-next v2 4/8] xsk: make xskmap flush

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Date:   Wed, 08 Jan 2020 17:01:08 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>,
        Björn Töpel 
        <bjorn.topel@...il.com>, John Fastabend <john.fastabend@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        bpf <bpf@...r.kernel.org>, David Miller <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        "Karlsson\, Magnus" <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: [PATCH bpf-next v2 4/8] xsk: make xskmap flush_list common for all map instances

John Fastabend <john.fastabend@...il.com> writes:

> Björn Töpel wrote:
>> On Tue, 7 Jan 2020 at 18:54, John Fastabend <john.fastabend@...il.com> wrote:
>> >
>> > Björn Töpel wrote:
>> > > From: Björn Töpel <bjorn.topel@...el.com>
>> > >
>> > > The xskmap flush list is used to track entries that need to flushed
>> > > from via the xdp_do_flush_map() function. This list used to be
>> > > per-map, but there is really no reason for that. Instead make the
>> > > flush list global for all xskmaps, which simplifies __xsk_map_flush()
>> > > and xsk_map_alloc().
>> > >
>> > > Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>
>> > > Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
>> > > ---
>> >
>> > Just to check. The reason this is OK is because xdp_do_flush_map()
>> > is called from NAPI context and is per CPU so the only entries on
>> > the list will be from the current cpu napi context?
>> 
>> Correct!
>> 
>> > Even in the case
>> > where multiple xskmaps exist we can't have entries from more than
>> > a single map on any list at the same time by my reading.
>> >
>> 
>> No, there can be entries from different (XSK) maps. Instead of
>> focusing on maps to flush, focus on *entries* to flush. At the end of
>> the poll function, all entries (regardless of map origin) will be
>> flushed. Makes sense?
>
> Ah OK. This would mean that a single program used multiple maps
> though correct? Because we can only run a single BPF program per
> NAPI context.

Yeah, there's nothing limiting each program to a single map (of any
type)...

-Toke