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SubjectRe: [PATCH 2/2] ARM: copypage: do not use naked functions
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.

Having this half-way house which will generate worse code is not
acceptable.

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