linux-kernel - Re: [PATCH] arm64: Enable CONFIG

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Date:	Thu, 4 Dec 2014 15:48:50 -0800
From:	Olof Johansson <olof@...om.net>
To:	Alexander Graf <agraf@...e.de>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH] arm64: Enable CONFIG_COMPAT also for 64k page size

On Thu, Dec 4, 2014 at 3:41 PM, Alexander Graf <agraf@...e.de> wrote:
>
>
> On 04.12.14 22:15, Olof Johansson wrote:
>> On Thu, Dec 4, 2014 at 7:46 AM, Alexander Graf <agraf@...e.de> wrote:
>>> With binutils 2.25 the default alignment for 32bit arm sections changed to
>>> have everything 64k aligned. Armv7 binaries built with this binutils version
>>> run successfully on an arm64 system.
>>>
>>> Since effectively there is now the chance to run armv7 code on arm64 even
>>> with 64k page size, it doesn't make sense to block people from enabling
>>> CONFIG_COMPAT on those configurations.
>>>
>>> Signed-off-by: Alexander Graf <agraf@...e.de>
>>> ---
>>>  arch/arm64/Kconfig | 1 -
>>>  1 file changed, 1 deletion(-)
>>>
>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>> index 9532f8d..3cf4f238 100644
>>> --- a/arch/arm64/Kconfig
>>> +++ b/arch/arm64/Kconfig
>>> @@ -409,7 +409,6 @@ source "fs/Kconfig.binfmt"
>>>
>>>  config COMPAT
>>>         bool "Kernel support for 32-bit EL0"
>>> -       depends on !ARM64_64K_PAGES
>>>         select COMPAT_BINFMT_ELF
>>>         select HAVE_UID16
>>>         select OLD_SIGSUSPEND3
>>
>> This is hardly "compat". Sure, it's great to have a new binutils that
>> has larger alignment, but practically not a single existing binary
>> will work today if someone tries to do this.
>
> Yes, but IMHO that's an implementation detail. The same applies for
> 32bit PPC binaries if you use 4k aligned segments. If your applications
> are not aligned for your page size, you can't run them. The only
> platform that managed nevertheless FWIW was IA64 ;).

Yes, but there the binutils change happened early enough that by the
time the kernel change went in, all major distros had binaries that
were compatible.

>> So, it seems very premature to take this off. At the very least
>> document it like Will requested, and make it depend on !ARM_64K_PAGES
>> || EXPERT.
>
> Sure, that certainly works for me (though you probably mean Laura).

Yeah, sorry Laura. :)

> Completely restricting users from enabling it just because our current
> toolchain doesn't generate compatible binaries sounds a bit extreme to me.

Perhaps.

It's in many ways a timing thing too, if ARM had patched binutils a
couple of years ago it might have been fine to just go with it without
EXPERT now. :)


-Olof
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