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

From: Russell King - ARM Linux
Date: Wed Nov 19 2014 - 11:49:19 EST

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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