On Sat, 5 Aug 2017, 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 > > 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. > > 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. There is no adrm_l definition in this patch. Also, might it be better to change mov_l to movl? Tthis looks similar to the ARM64 movl pseudo-instruction, and unlike all the other _l variants, this is not producing a pc relative result. Talking about the _l suffix: I wonder if this could be more meaningful, like _rel maybe? At least in the adr_l case, this could easily be confused with adrl. Otherwise I like it pretty much. > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> > --- > arch/arm/include/asm/assembler.h | 70 ++++++++++++++++++++ > 1 file changed, 70 insertions(+) > > diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h > index ad301f107dd2..cedf59a7f853 100644 > --- a/arch/arm/include/asm/assembler.h > +++ b/arch/arm/include/asm/assembler.h > @@ -518,4 +518,74 @@ 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 > + .if __LINUX_ARM_ARCH__ < 7 > + ldr \tmp, 111f > + .subsection 1 > + .align 2 > +111: .long \sym - (222f + ARM_PC_BIAS) > + .previous > + .else > + movw \tmp, #:lower16:\sym - (222f + ARM_PC_BIAS) > + movt \tmp, #:upper16:\sym - (222f + ARM_PC_BIAS) > + .endif > +222:; .ifc \op, add > + add \reg, \tmp, pc > + .elseif CONFIG_THUMB2_KERNEL == 1 > + add \tmp, \tmp, pc > + \op \reg, [\tmp] > + .else > + \op \reg, [pc, \tmp] > + .endif > + .endm > + > + /* > + * mov_l - move a constant value or [relocated] address into a register > + */ > + .macro mov_l, dst:req, imm:req > + .if __LINUX_ARM_ARCH__ < 7 > + ldr \dst, =\imm > + .else > + movw \dst, #:lower16:\imm > + movt \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 > + __adldst_l add, \dst, \sym, \dst > + .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 > + __adldst_l ldr, \dst, \sym, \dst > + .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 > + __adldst_l str, \src, \sym, \tmp > + .endm > + > #endif /* __ASM_ASSEMBLER_H__ */ > -- > 2.11.0 > >
