linux-kernel - Re: [PATCH RFC] ARM: option for loading modules into vmalloc area

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Message-ID: <CAKv+Gu_USuMfNWfxcj6fSztOva5614v1YTQxFTonh6KRkKvUGg@mail.gmail.com>
Date:	Wed, 19 Nov 2014 17:41:53 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Konstantin Khlebnikov <k.khlebnikov@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH RFC] ARM: option for loading modules into vmalloc area

On 19 November 2014 17:37, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> On Wed, 19 Nov 2014, Russell King - ARM Linux wrote:
>
>> On Wed, Nov 19, 2014 at 05:02:40PM +0100, Ard Biesheuvel wrote:
>> > On 19 November 2014 16:52, Konstantin Khlebnikov <koct9i@...il.com> wrote:
>> > > Do you mean ldr pc, =symbol ?
>> > >
>> > > In this case I get this error:
>> > >
>> > > /tmp/ccAHtONU.s: Assembler messages:
>> > > /tmp/ccAHtONU.s:220: Error: invalid literal constant: pool needs to be closer
>> > >
>> > > Probably constant pool doesn't work well in inline assembly.
>> > >
>> > >
>> > > Something like this seems work:
>> > >
>> > > add     lr, pc, #4
>> > > ldr       pc, [pc, #-4]
>> > > .long symbol
>> > >
>> >
>> > You can add a '.ltorg' instruction which tells the assembler to dump
>> > the literal pool, but you still need to jump over it, i.e.,
>> >
>> > adr lr, 0f
>> > ldr pc, =symbol
>> > .ltorg
>> > 0:
>>
>> Which is not a good idea either, because the compiler needs to know how
>> far away its own manually generated literal pool is from the instructions
>> which reference it.  The .ltorg statement can end up emitting any number
>> of literals at that point, which makes it indeterminant how many words
>> are contained within the asm() statement.
>>
>> Yes, it isn't desirable to waste an entire data cache line per indirect
>> call like the original quote above, but I don't see a practical
>> alternative.
>
> Modules could be built without far calls by default, and then the module
> linker would only have to redirect those calls whose destination is too
> far away to a dynamically created trampoline table.
>
> If I remember correctly you even posted some patches to that effect a
> couple years ago.  Maybe those could be salvaged?
>
> I would largely recommend a solution where the link process could deal
> with it automatically and as needed rather than sprinkling yet more
> manually maintained macros into assembly code.
>

Yes, that would be preferable. I played around with 'bl symbol@...'
instead of plain 'bl symbol' but unfortunately, our .ko's are not
proper ELF shared libraries, so that doesn't work.
But essentially, we just need a (eager) PLT.

-- 
Ard.
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