Re: [PATCH 1/2] ARM: copypage-fa: add kto and kfrom to input operands list

From: Arnd Bergmann
Date: Wed Oct 17 2018 - 04:59:13 EST

On Tue, Oct 16, 2018 at 10:43 PM Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> wrote:
> On Tue, 16 Oct 2018, Russell King - ARM Linux wrote:
> > On Tue, Oct 16, 2018 at 10:00:19AM +0200, Linus Walleij wrote:
> > > On Tue, Oct 16, 2018 at 12:16 AM Stefan Agner <stefan@xxxxxxxx> wrote:
> > It's not obvious yet whether this is right - it contradicts the GCC
> > manual, but then we have evidence that it's required for some GCC
> > versions where GCC may clone the function, or if the function is
> > used within the same file.
> Why not getting rid of __naked altogether? Here's what I suggest:
> ----- >8
> Subject: [PATCH] ARM: remove naked function usage
> Convert page copy functions not to rely on the naked function attribute.
> This attribute is known to confuse some gcc versions when function
> arguments aren't explicitly listed as inline assembly operands despite
> the gcc documentation. That resulted in commit 9a40ac86152c ("ARM:
> 6164/1: Add kto and kfrom to input operands list.").

It's probably worth noting that the minimum gcc version for compiling
the kernel is now gcc-4.6, which I think does not suffer from the gcc-4.5
bug that triggered the change. See in particular commits 9c695203a7dd
("compiler-gcc.h: gcc-4.5 needs noclone and noinline on __naked functions")
and d124b44f09ca ("Compiler Attributes: naked was fixed in gcc 4.6").

The first one made sure we don't inline these functions, so gcc-4.5
no longer runs into the problem even in the absence of the workaround,
and the second patch reverts that again, noting that gcc-4.6 is fixed.

I don't see anything wrong with converting the functions to not
use __naked at all, but I think we can also just revert the original
commit 9a40ac86152c to get it to build with clang. When I last
played with clang on arm32, that's what I did. I'll reply with the
patch I have in my randconfig tree.