Re: [PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'
From: Segher Boessenkool <hidden>
Date: 2020-05-06 17:59:16
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On Wed, May 06, 2020 at 10:58:55AM +1000, Michael Ellerman wrote:
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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?
Nope. But 4.6.4 is a bug-fix release, 91 bugs fixed since 4.6.3, so you should switch to it if you can :-)
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
Yes, I know. And it is much preferred not to have stricter requirements for Power, I know that too. Something has to give though :-/
There were discussions about making 4.8 the minimum, but I'm not sure where they got to.
Yeah, just petered out I think? All significant distros come with a 4.8 as system compiler.
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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)
Like that. But you will have to do that to *all* places we use the "<>" constraints, or wait for more stuff to fail? And, there probably are places we *do* want update form insns used (they do help in some loops, for example)? Segher