From: ard.biesheuvel@linaro.org (Ard Biesheuvel)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH RFC] ARM: option for loading modules into vmalloc area
Date: Wed, 19 Nov 2014 17:38:57 +0100 [thread overview]
Message-ID: <CAKv+Gu8o_-Q5PeFqdQXW=mcHNYxFcLLnvxjq94y-iG8+JUp=9w@mail.gmail.com> (raw)
In-Reply-To: <CALYGNiN5XC_4MhRbxZmFW5Jmc1N1H32tSZr3kKf_UBFriG71nA@mail.gmail.com>
On 19 November 2014 17:32, Konstantin Khlebnikov <koct9i@gmail.com> wrote:
> 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.
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.
next prev parent reply other threads:[~2014-11-19 16:38 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-11-18 16:21 [PATCH RFC] ARM: option for loading modules into vmalloc area Konstantin Khlebnikov
2014-11-18 17:34 ` Russell King - ARM Linux
2014-11-18 18:13 ` Konstantin Khlebnikov
2014-11-19 13:40 ` Arnd Bergmann
2014-11-19 14:54 ` Ard Biesheuvel
2014-11-19 15:52 ` Konstantin Khlebnikov
2014-11-19 16:02 ` Ard Biesheuvel
2014-11-19 16:07 ` Russell King - ARM Linux
2014-11-19 16:25 ` Ard Biesheuvel
2014-11-19 16:32 ` Konstantin Khlebnikov
2014-11-19 16:38 ` Ard Biesheuvel [this message]
2014-11-19 16:55 ` Russell King - ARM Linux
2014-11-19 16:59 ` Nicolas Pitre
2014-11-19 16:41 ` Russell King - ARM Linux
2014-11-19 16:37 ` Nicolas Pitre
2014-11-19 16:41 ` Ard Biesheuvel
2014-11-19 16:49 ` Russell King - ARM Linux
2014-11-19 16:57 ` Nicolas Pitre
2014-11-19 16:59 ` Russell King - ARM Linux
2014-11-19 17:12 ` Nicolas Pitre
2014-11-19 17:59 ` Ard Biesheuvel
2014-11-19 18:22 ` Nicolas Pitre
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