linux-kernel - Re: [PATCH v3] Add BPF_SYNCHRONIZE_MAP_TO_MAP

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Date:   Mon, 30 Jul 2018 17:26:41 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Joel Fernandes <joelaf@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Lorenzo Colitti <lorenzo@...gle.com>,
        Chenbo Feng <fengc@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Alexei Starovoitov <ast@...com>
Subject: Re: [PATCH v3] Add BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES bpf(2)
 command

On Mon, 30 Jul 2018 03:25:43 -0700, Daniel Colascione wrote:
> On Mon, Jul 30, 2018 at 3:04 AM, Daniel Borkmann <daniel@...earbox.net> wrote:
> > Hmm, I don't think such UAPI as above is future-proof. In case we would want
> > a similar mechanism in future for other maps, we would need a whole new bpf
> > command or reuse BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES as a workaround though
> > the underlying map may not even be a map-to-map. Additionally, we don't have
> > any map object at hand in the above, so we couldn't make any finer grained
> > decisions either. Something like below would be more suitable and leaves room
> > for extending this further in future.  
> 
> YAGNI.  Your proposed mechanism doesn't add anything under the current
> implementation. 

FWIW in case of HW offload targeting a particular map may allow users
to avoid a potentially slow sync with all the devices on the system.

> It's also not clear how a map-specific synchronization
> command is supposed to work in cases where we swap multiple map
> references. Do we synchronize_rcu multiple times? Why would we impose
> that inefficiency just for the sake of some non-specific future
> extensibility? Add some kind of batching layer? The current approach
> works for the anticipated use cases.
> 
> While my preference is not to talk about map-to-maps at all in the
> user API and instead spec the thing as talking about map references in
> general, I'd rather have something that talks about
> references-to-maps-acquired-from-maps than this interface.