Re: [PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'

Michael Ellerman
2020-05-06T00:58:42Z

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