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

[Nicolas Pitre]

On Wed, 19 Nov 2014, Russell King - ARM Linux wrote:

> 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.

Still... We could try adding a hook in the generic module linker code 
for a pre-relocation pass.  Maybe only ARM would use it, but if the need 
to load big modules is real then I imagine Linus could be amenable to a 
compromise.


Nicolas
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