Messages in this thread | | | Date | Wed, 19 Nov 2014 20:32:27 +0400 | Subject | Re: [PATCH RFC] ARM: option for loading modules into vmalloc area | From | Konstantin Khlebnikov <> |
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On Wed, Nov 19, 2014 at 7:25 PM, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote: > On 19 November 2014 17:07, Russell King - ARM Linux > <linux@arm.linux.org.uk> wrote: >> On Wed, Nov 19, 2014 at 05:02:40PM +0100, Ard Biesheuvel wrote: >>> On 19 November 2014 16:52, Konstantin Khlebnikov <koct9i@gmail.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. >> > > That applies to any inline asm statement in general: the compiler > assumes that the expanded size will not interfere with its ability to > emit literals after the function's return instruction. > Sometimes it will put a literal pool in the middle of the function if > it is very large, and I am not sure if an inline asm by itself would > ever trigger that heuristic to kick in. > > But by the same logic, i.e., due to the fact that GCC manages its own > literals, the literal pool at the assembly level is unlikely to be so > large that you will actually hit this condition. > >> 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. >> > > We could at least add some labels instead of doing explicit pc arithmetic, i.e., > > adr lr, 1f > ldr pc, 0f > 0: .long symbol > 1:
I think we need some unique prefix here, this macro is used inside bigger inline assembly constructions and probably another macro.
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