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

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Message-ID: <20141119164902.GK4042@n2100.arm.linux.org.uk>
Date:	Wed, 19 Nov 2014 16:49:02 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
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, Nov 19, 2014 at 11:37:47AM -0500, Nicolas Pitre wrote:
> On Wed, 19 Nov 2014, Russell King - ARM Linux wrote:
> > 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 don't think I ever did, because its pretty much impossible to do as I
explained in a follow up to this thread.

We _used_ to do this with the userspace insmod methods, but since we got
this kernel-side linker, it's been pretty much impossible to do without
rewriting the module code.  That's not going to happen on account of one
quirky architecture which Linus doesn't particularly like.

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