Messages in this thread | | | Date | Tue, 16 Oct 2018 00:51:26 +0200 | From | Stefan Agner <> | Subject | Re: [PATCH 2/2] ARM: copypage: do not use naked functions |
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On 16.10.2018 00:41, Russell King - ARM Linux wrote: > On Mon, Oct 15, 2018 at 06:35:33PM -0400, Nicolas Pitre wrote: >> On Tue, 16 Oct 2018, Stefan Agner wrote: >> >> > GCC documentation says naked functions should only use basic ASM >> > syntax. The extended ASM or mixture of basic ASM and "C" code is >> > not guaranteed. Currently it seems to work though. >> > >> > Furthermore with Clang using parameters in extended asm in a >> > naked function is not supported: >> > arch/arm/mm/copypage-v4wb.c:47:9: error: parameter references not >> > allowed in naked functions >> > : "r" (kto), "r" (kfrom), "I" (PAGE_SIZE / 64)); >> > ^ >> > >> > Use a regular function to be more portable. Also use volatile asm >> > to avoid unsolicited optimizations. >> > >> > Tested with qemu versatileab machine and versatile_defconfig and >> > qemu mainstone machine using pxa_defconfig compiled with GCC 7.2.1 >> > and Clang 7.0. >> > >> > Link: https://github.com/ClangBuiltLinux/linux/issues/90 >> > Reported-by: Joel Stanley <joel@jms.id.au> >> > Signed-off-by: Stefan Agner <stefan@agner.ch> >> > --- >> > arch/arm/mm/copypage-fa.c | 17 +++++++++++------ >> > arch/arm/mm/copypage-feroceon.c | 17 +++++++++++------ >> > arch/arm/mm/copypage-v4mc.c | 14 +++++++++----- >> > arch/arm/mm/copypage-v4wb.c | 17 +++++++++++------ >> > arch/arm/mm/copypage-v4wt.c | 17 +++++++++++------ >> > arch/arm/mm/copypage-xsc3.c | 17 +++++++++++------ >> > arch/arm/mm/copypage-xscale.c | 13 ++++++++----- >> > 7 files changed, 72 insertions(+), 40 deletions(-) >> > >> > diff --git a/arch/arm/mm/copypage-fa.c b/arch/arm/mm/copypage-fa.c >> > index ec6501308c60..33ccd396bf99 100644 >> > --- a/arch/arm/mm/copypage-fa.c >> > +++ b/arch/arm/mm/copypage-fa.c >> > @@ -17,11 +17,16 @@ >> > /* >> > * Faraday optimised copy_user_page >> > */ >> > -static void __naked >> > -fa_copy_user_page(void *kto, const void *kfrom) >> > +static void fa_copy_user_page(void *kto, const void *kfrom) >> > { >> > - asm("\ >> > - stmfd sp!, {r4, lr} @ 2\n\ >> > + register void *r0 asm("r0") = kto; >> > + register const void *r1 asm("r1") = kfrom; >> > + >> > + asm( >> > + __asmeq("%0", "r0") >> > + __asmeq("%1", "r1") >> > + "\ >> > + stmfd sp!, {r4} @ 2\n\ >> > mov r2, %2 @ 1\n\ >> > 1: ldmia r1!, {r3, r4, ip, lr} @ 4\n\ >> > stmia r0, {r3, r4, ip, lr} @ 4\n\ >> > @@ -34,9 +39,9 @@ fa_copy_user_page(void *kto, const void *kfrom) >> > subs r2, r2, #1 @ 1\n\ >> > bne 1b @ 1\n\ >> > mcr p15, 0, r2, c7, c10, 4 @ 1 drain WB\n\ >> > - ldmfd sp!, {r4, pc} @ 3" >> > + ldmfd sp!, {r4} @ 3" >> > : >> > - : "r" (kto), "r" (kfrom), "I" (PAGE_SIZE / 32)); >> > + : "r" (r0), "r" (r1), "I" (PAGE_SIZE / 32)); >> >> This is still wrong as you list r0 and r1 in the input operand list >> where they must remain constant but the code does modify them. You >> should list them in the output operand list with the "&" attribute. Also >> r2 should be listed in the clobbered list. > > Either we keep these as naked functions (and, if Clang wants to > try to inline naked functions which makes no sense, also mark them > as noinline) or we make them proper functions and also add (eg) r4 > to the clobber list and get rid of the stacking of that register > along with LR/PC.
Clang does not inline naked functions, at least that is what a quick look at the disassembled code shows when compiling with 9a40ac86152c reverted.
> > Having this half-way house which will generate worse code is not > acceptable.
For Clang reverting 9a40ac86152c ("ARM: 6164/1: Add kto and kfrom to input operands list.") is a solution...
I guess the question is why that commit was necessary back then... Do we break something by reverting it?
-- Stefan
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