Segher Boessenkool [off-list ref] writes:

Hi!

On Wed, May 06, 2020 at 12:27:58AM +1000, Michael Ellerman wrote:

quoted

Christophe Leroy [off-list ref] writes:

quoted

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()

...

quoted

Signed-off-by: Christophe Leroy <redacted>
---
 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.

quoted

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