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

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Date:	Wed, 19 Nov 2014 11:37:47 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	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 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.


Nicolas
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