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

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Date:	Wed, 19 Nov 2014 20:32:27 +0400
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Konstantin Khlebnikov <k.khlebnikov@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH RFC] ARM: option for loading modules into vmalloc area

On Wed, Nov 19, 2014 at 7:25 PM, Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
> On 19 November 2014 17:07, Russell King - ARM Linux
> <linux@....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@...il.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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