netdev - Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin

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Date:   Fri, 25 Jan 2019 11:23:12 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>, davem@...emloft.net,
        daniel@...earbox.net, jakub.kicinski@...ronome.com,
        netdev@...r.kernel.org, kernel-team@...com, mingo@...hat.com,
        will.deacon@....com, Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        jannh@...gle.com
Subject: Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin_lock

On Thu, Jan 24, 2019 at 03:58:59PM -0800, Alexei Starovoitov wrote:
> On Thu, Jan 24, 2019 at 07:01:09PM +0100, Peter Zijlstra wrote:

> > And this would again be the moment where I go pester you about the BPF
> > memory model :-)
> 
> hehe :)
> How do you propose to define it in a way that it applies to all archs
> and yet doesn't penalize x86 ?
> "Assume minimum execution ordering model" the way kernel does
> unfortunately is not usable, since bpf doesn't have a luxury
> of using nice #defines that convert into nops on x86.

Why not? Surely the JIT can fix it up? That is, suppose you were to have
smp_rmb() as a eBPF instruction then the JIT (which knows what
architecture it is targeting) can simply avoid emitting instructions for
it.

Similarly; could something like this not also help with the spinlock
thing? Use that generic test-and-set thing for the interpreter, but
provide a better implementation in the JIT?