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

[Ard Biesheuvel]

On 19 November 2014 16:52, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
> On Wed, Nov 19, 2014 at 5:54 PM, Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
>> On 19 November 2014 14:40, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>>> On Tuesday 18 November 2014 21:13:56 Konstantin Khlebnikov wrote:
>>>> On 2014-11-18 20:34, Russell King - ARM Linux wrote:
>>>> > On Tue, Nov 18, 2014 at 08:21:46PM +0400, Konstantin Khlebnikov wrote:
>>>> >> Usually modules are loaded into small area prior to the kernel
>>>> >> text because they are linked with the kernel using short calls.
>>>> >> Compile-time instrumentation like GCOV or KASAN bloats code a lot,
>>>> >> and as a result huge modules no longer fit into reserved area.
>>>> >>
>>>> >> This patch adds option CONFIG_MODULES_USE_VMALLOC which lifts
>>>> >> limitation on amount of loaded modules. It links modules using
>>>> >> long-calls (option -mlong-calls) and loads them into vmalloc area.
>>>> >>
>>>> >> In few places exported symbols are called from inline assembly.
>>>> >> This patch adds macro for such call sites: __asmbl and __asmbl_clobber.
>>>> >> Call turns into single 'bl' or sequence 'movw; movt; blx' depending on
>>>> >> context and state of config option.
>>>> >>
>>>> >> Unfortunately this option isn't compatible with CONFIG_FUNCTION_TRACER.
>>>> >> Compiler emits short calls to profiling function despite of -mlong-calls.
>>>> >> This is a bug in GCC, but ftrace anyway needs an update to handle this.
>>>> > It also isn't compatible with the older architectures which don't have
>>>> > "blx".
>>>>
>>>> Ok, I'll add "depends on CPU_V6 || CPU_V7" I don't think that it is
>>>> necessary for older cpus.
>>>
>>> Why not just use a different branch instruction for the older CPUs?
>>>
>>
>> ARMv6 doesn't support movw/movt so this will only work on v7.
>>
>> What about doing 'mov lr, pc; ldr pc,=symbol' instead? You clearly
>> don't care about performance in this case, so the performance hit (due
>> to the dcache access and interfering with the return stack predictors)
>> should be tolerable. The only thing to be careful about is thumb2
>> kernels: you would need to set the thumb bit in lr manually but only
>> if the call is made /from/ thumb. You would probably be better off
>> just depending on !THUMB2_KERNEL.
>
> 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:
-- 
Ard.
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