linux-kernel - Re: [PATCH v5 00/23] ILP32 for ARM64

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Message-ID: <20151005155857.GA5557@MBP.local>
Date:	Mon, 5 Oct 2015 16:58:59 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	"Kapoor, Prasun" <Prasun.Kapoor@...iumnetworks.com>
Cc:	"Pinski, Andrew" <Andrew.Pinski@...iumnetworks.com>,
	Arnd Bergmann <arnd@...db.de>,
	"yury.norov@...il.com" <yury.norov@...il.com>,
	"Norov, Yuri" <Yuri.Norov@...iumnetworks.com>,
	"agraf@...e.de" <agraf@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"klimov.linux@...il.com" <klimov.linux@...il.com>,
	"bamvor.zhangjian@...wei.com" <bamvor.zhangjian@...wei.com>,
	"apinski@...ium.com" <apinski@...ium.com>,
	"philipp.tomsich@...obroma-systems.com" 
	<philipp.tomsich@...obroma-systems.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"christoph.muellner@...obroma-systems.com" 
	<christoph.muellner@...obroma-systems.com>
Subject: Re: [PATCH v5 00/23] ILP32 for ARM64

On Sat, Oct 03, 2015 at 02:18:57AM +0000, Kapoor, Prasun wrote:
> On 10/2/15, 2:37 AM, "Catalin Marinas" <catalin.marinas@....com> wrote:
> >So, at the time, following x32 discussions, we thought of using the
> >native ABI as much as possible. However, two important things happened
> >since:
> >
> >1. libc community didn't like breaking the POSIX compliance
> >2. No-one seems desperate for ILP32 on AArch64
> >
> >(1) is a fair point and I would rather be careful as we don't know the
> >extent of the code affected. In the meantime, we've also had ongoing
> >work for addressing the 2038 issue on 32-bit architectures.
> >
> >The second point is equally important. The benchmarks I've seen didn't
> >show a significant improvement and the messages I got on various
> >channels pretty much labeled ILP32 as a transitional stage to full LP64.
> >In this case, we need to balance the benefits of a close to native ABI
> >(future proof, slightly higher performance) vs. the cost of maintaining
> >such ABI in the kernel on the long term, especially if it's not widely
> >used/tested.
> 
> For us ILP32  is not about putting this into our product flier at all, it
> is about supporting real applications. We have an existing product line of
> MIPS based SoCs where a large number of N32 (an exact equivalent of ILP32)
> applications are currently in production. Our customers are looking to
> bring those applications (mostly in Networking and Telecom space) over to
> ARMv8. 
> 
> We think its an extremely risky strategy to say either future processors
> should incur the additional cost (power and complexity) of implementing
> Aarch32 instruction set or have no way of  supporting 32 bit applications
> at all.

Well, given that Cavium posted only 3 versions of this series since
September 2013, it doesn't seem critical at all.

> Apart from there being an installed base of 32 bit networking and telecom
> applications, we have also seen non-trivial performance gains with ILP32
> (for example, our SPECINT score goes up by 7% with ILP32 compared to
> LP64).

It would be good to re-run the benchmarks with the latest gcc since
LP64/AArch64 support has evolved in the meantime.

-- 
Catalin
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