[PATCH 02/30] ARM: assembler: introduce adr_l, ldr_l and str_l macros
Ard Biesheuvel
ard.biesheuvel at linaro.org
Mon Aug 14 08:38:02 PDT 2017
On 14 August 2017 at 16:29, Dave Martin <Dave.Martin at arm.com> wrote:
> On Mon, Aug 14, 2017 at 01:53:43PM +0100, Ard Biesheuvel wrote:
>> Like arm64, ARM supports position independent code sequences that
>> produce symbol references with a greater reach than the ordinary
>> adr/ldr instructions.
>>
>> Currently, we use open coded instruction sequences involving literals
>> and arithmetic operations. Instead, we can use movw/movt pairs on v7
>> CPUs, circumventing the D-cache entirely. For older CPUs, we can emit
>> the literal into a subsection, allowing it to be emitted out of line
>> while retaining the ability to perform arithmetic on label offsets.
>>
>> E.g., on pre-v7 CPUs, we can emit a PC-relative reference as follows:
>>
>> ldr <reg>, 222f
>> 111: add <reg>, <reg>, pc
>> .subsection 1
>> 222: .long <sym> - (111b + 8)
>> .previous
>>
>> This is allowed by the assembler because, unlike ordinary sections,
>> subsections are combined into a single section into the object file,
>> and so the label references are not true cross-section references that
>> are visible as relocations. Note that we could even do something like
>>
>> add <reg>, pc, #(222f - 111f) & ~0xfff
>> ldr <reg>, [<reg>, #(222f - 111f) & 0xfff]
>> 111: add <reg>, <reg>, pc
>> .subsection 1
>> 222: .long <sym> - (111b + 8)
>> .previous
>
> This is reinventing ldr=
>
> I seem to remember ldr= barfing on things that .long happily accepts
> though, was this the reason?
>
Yes. ldr = does not accept expressions involving symbols, only plain
symbols or expressions that evaluate to constants.
So something like
ldr <reg>, =<sym> - <label>
is rejected while the equivalent
ldr <reg>, 0f
0: .long <sym> - <label>
does work.
>> if it turns out that the 4 KB range of the ldr instruction is insufficient
>> to reach the literal in the subsection, although this is currently not a
>> problem (of the 98 objects built from .S files in a multi_v7_defconfig
>> build, only 11 have .text sections that are over 1 KB, and the largest one
>> [entry-armv.o] is 3308 bytes)
>>
>> Subsections have been available in binutils since 2004 at least, so
>> they should not cause any issues with older toolchains.
>
> (I also believe this to be an ancient feature, but I've not done the
> digging to prove it.)
>
OK
>> So use the above to implement the macros mov_l, adr_l, adrm_l (using ldm
>> to load multiple literals at once), ldr_l and str_l, all of which will
>> use movw/movt pairs on v7 and later CPUs, and use PC-relative literals
>> otherwise.
>>
>> Cc: Russell King <linux at armlinux.org.uk>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> ---
>> arch/arm/include/asm/assembler.h | 71 ++++++++++++++++++++
>> 1 file changed, 71 insertions(+)
>>
>> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
>> index ad301f107dd2..516ebaf4ff38 100644
>> --- a/arch/arm/include/asm/assembler.h
>> +++ b/arch/arm/include/asm/assembler.h
>> @@ -518,4 +518,75 @@ THUMB( orr \reg , \reg , #PSR_T_BIT )
>> #endif
>> .endm
>>
>> +#ifdef CONFIG_THUMB2_KERNEL
>> +#define ARM_PC_BIAS 4
>> +#else
>> +#define ARM_PC_BIAS 8
>> +#endif
>> +
>> + .macro __adldst_l, op, reg, sym, tmp, c
>> + .if __LINUX_ARM_ARCH__ < 7
>> + ldr\c \tmp, 111f
>> + .subsection 1
>> + .align 2
>> +111: .long \sym - (222f + ARM_PC_BIAS)
>
> See above comment about ldr=.
>
>> + .previous
>> + .else
>> + W(movw\c\()) \tmp, #:lower16:\sym - (222f + ARM_PC_BIAS)
>> + W(movt\c\()) \tmp, #:upper16:\sym - (222f + ARM_PC_BIAS)
>
> Why W()?
>
> There are no narrow forms of these instructions anyway -- if there were
> then they couldn't accommodate a 16-bit immediate.
>
That's a trick, actually, which I failed to add a comment for.
We use .arm sections in the thumb2 kernel, and using these macros
there would result in the wrong offset to be used. Adding the .w
suffix forces an error in the assembler which even results in a fairly
meaningful error message complaining about using .w in ARM code.
>> + .endif
>> +222:
>> + .ifc \op, add
>> + add\c \reg, \tmp, pc
>> + .elseif CONFIG_THUMB2_KERNEL == 1
>> + add \tmp, \tmp, pc
>> + \op\c \reg, [\tmp]
>
> Shame
> \op\c \reg, [pc, \tmp]
> doesn't work.
>
> But it doesn't, apparently.
>
No, thumb2 does not allow that
>> + .else
>> + \op\c \reg, [pc, \tmp]
>> + .endif
>> + .endm
>> +
>> + /*
>> + * mov_l - move a constant value or [relocated] address into a register
>> + */
>> + .macro mov_l, dst:req, imm:req, cond
>> + .if __LINUX_ARM_ARCH__ < 7
>> + ldr\cond \dst, =\imm
>> + .else
>> + W(movw\cond\()) \dst, #:lower16:\imm
>> + W(movt\cond\()) \dst, #:upper16:\imm
>> + .endif
>> + .endm
>> +
>> + /*
>> + * adr_l - adr pseudo-op with unlimited range
>> + *
>> + * @dst: destination register
>> + * @sym: name of the symbol
>> + */
>> + .macro adr_l, dst:req, sym:req, cond
>> + __adldst_l add, \dst, \sym, \dst, \cond
>> + .endm
>> +
>> + /*
>> + * ldr_l - ldr <literal> pseudo-op with unlimited range
>> + *
>> + * @dst: destination register
>> + * @sym: name of the symbol
>> + */
>> + .macro ldr_l, dst:req, sym:req, cond
>> + __adldst_l ldr, \dst, \sym, \dst, \cond
>> + .endm
>> +
>> + /*
>> + * str_l - str <literal> pseudo-op with unlimited range
>> + *
>> + * @src: source register
>> + * @sym: name of the symbol
>> + * @tmp: mandatory scratch register
>> + */
>> + .macro str_l, src:req, sym:req, tmp:req, cond
>> + __adldst_l str, \src, \sym, \tmp, \cond
>> + .endm
>
> Cheers
> ---Dave
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