netdev - Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction

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Date:	Wed, 11 Nov 2015 19:41:34 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	David Miller <davem@...emloft.net>, will.deacon@....com,
	daniel@...earbox.net, arnd@...db.de, yang.shi@...aro.org,
	linaro-kernel@...ts.linaro.org, eric.dumazet@...il.com,
	zlim.lnx@...il.com, ast@...nel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, xi.wang@...il.com, catalin.marinas@....com,
	linux-arm-kernel@...ts.infradead.org, yhs@...mgrid.com,
	bblanco@...mgrid.com
Subject: Re: [PATCH 2/2] arm64: bpf: add BPF XADD instruction

On Wed, Nov 11, 2015 at 07:31:28PM +0100, Peter Zijlstra wrote:
> > Adding new intrinsic to llvm is not a big deal. I'll add it as soon
> > as I have time to work on it or if somebody beats me to it I would be
> > glad to test it and apply it.
> 
> This isn't a speed coding contest. You want to think about this
> properly.

That is, I don't think you want to go add LLVM intrinsics at all. You
want to piggy back on the memory model work done by the C/C++11 people.

What you want to think about is what the memory model of your virtual
machine is and how many instructions you want to expose for that.

Concurrency is a right pain, a little time and effort now will safe
heaps of pain down the road.
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