Messages in this thread |  | | Subject | Re: [PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put_user() using 'asm goto' | From | Christophe Leroy <> | Date | Tue, 5 May 2020 17:40:21 +0200 |
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Hi,
Le 05/05/2020 à 16:27, Michael Ellerman a écrit : > Christophe Leroy <christophe.leroy@c-s.fr> writes: >> unsafe_put_user() is designed to take benefit of 'asm goto'. >> >> Instead of using the standard __put_user() approach and branch >> based on the returned error, use 'asm goto' and make the >> exception code branch directly to the error label. There is >> no code anymore in the fixup section. >> >> This change significantly simplifies functions using >> unsafe_put_user() >> > ... >> >> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr> >> --- >> arch/powerpc/include/asm/uaccess.h | 61 +++++++++++++++++++++++++----- >> 1 file changed, 52 insertions(+), 9 deletions(-) >> >> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h >> index 9cc9c106ae2a..9365b59495a2 100644 >> --- a/arch/powerpc/include/asm/uaccess.h >> +++ b/arch/powerpc/include/asm/uaccess.h >> @@ -196,6 +193,52 @@ do { \ >> }) >> >> >> +#define __put_user_asm_goto(x, addr, label, op) \ >> + asm volatile goto( \ >> + "1: " op "%U1%X1 %0,%1 # put_user\n" \ >> + EX_TABLE(1b, %l2) \ >> + : \ >> + : "r" (x), "m<>" (*addr) \ > > The "m<>" here is breaking GCC 4.6.3, which we allegedly still support. > > Plain "m" works, how much does the "<>" affect code gen in practice? > > A quick diff here shows no difference from removing "<>".
It was recommended by Segher, there has been some discussion about it on v1 of this patch, see https://patchwork.ozlabs.org/project/linuxppc-dev/patch/4fdc2aba6f5e51887d1cd0fee94be0989eada2cd.1586942312.git.christophe.leroy@c-s.fr/
As far as I understood that's mandatory on recent gcc to get the pre-update form of the instruction. With older versions "m" was doing the same, but not anymore. Should we ifdef the "m<>" or "m" based on GCC version ?
Christophe
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