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

[Ard Biesheuvel]

On 19 November 2014 17:32, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
> On Wed, Nov 19, 2014 at 7:25 PM, Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
>> On 19 November 2014 17:07, Russell King - ARM Linux
>> <linux@xxxxxxxxxxxxxxxx> wrote:
>>> On Wed, Nov 19, 2014 at 05:02:40PM +0100, Ard Biesheuvel wrote:
>>>> On 19 November 2014 16:52, Konstantin Khlebnikov <koct9i@xxxxxxxxx> 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.

Numbers are disambiguated by the f and b suffixes, so they can be
reused in the same .s file. So as long as you use a strictly numerical
prefix, you can deal correctly with the case where, for instance,
do_div() is called twice in the same compilation unit, and still not
clash with other inline asm

-- 
Ard.
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