Messages in this thread Patch in this message |  | | From | Michael Ellerman <> | Subject | Re: [PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put_user() using 'asm goto' | Date | Wed, 06 May 2020 10:58:55 +1000 |
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Segher Boessenkool <segher@kernel.crashing.org> writes: > Hi! > > On Wed, May 06, 2020 at 12:27:58AM +1000, Michael Ellerman wrote: >> 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. > > [ You shouldn't use 4.6.3, there has been 4.6.4 since a while. And 4.6 > is nine years old now. Most projects do not support < 4.8 anymore, on > any architecture. ]
Moving up to 4.6.4 wouldn't actually help with this though would it?
Also I have 4.6.3 compilers already built, I don't really have time to rebuild them for 4.6.4.
The kernel has a top-level minimum version, which I'm not in charge of, see:
https://www.kernel.org/doc/html/latest/process/changes.html?highlight=gcc
There were discussions about making 4.8 the minimum, but I'm not sure where they got to.
>> Plain "m" works, how much does the "<>" affect code gen in practice? >> >> A quick diff here shows no difference from removing "<>". > > It will make it impossible to use update-form instructions here. That > probably does not matter much at all, in this case. > > If you remove the "<>" constraints, also remove the "%Un" output modifier?
So like this?
diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h index 62cc8d7640ec..ca847aed8e45 100644 --- a/arch/powerpc/include/asm/uaccess.h +++ b/arch/powerpc/include/asm/uaccess.h @@ -207,10 +207,10 @@ do { \ #define __put_user_asm_goto(x, addr, label, op) \ asm volatile goto( \ - "1: " op "%U1%X1 %0,%1 # put_user\n" \ + "1: " op "%X1 %0,%1 # put_user\n" \ EX_TABLE(1b, %l2) \ : \ - : "r" (x), "m<>" (*addr) \ + : "r" (x), "m" (*addr) \ : \ : label)
cheers
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