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Re: [Xen-devel] [PATCH] arm32: fix build after 063188f4b3

On 10/10/2014 17:01, Ian Campbell wrote:
On Fri, 2014-10-10 at 16:55 +0100, Julien Grall wrote:
Hi Jan,

On 10/10/2014 15:51, Jan Beulich wrote:
On 10.10.14 at 16:12, <julien.grall@xxxxxxxxxx> wrote:
On 10/10/2014 14:58, Jan Beulich wrote:
"xen: arm: Add support for the Exynos secure firmware" introduced code
assuming that exynos_smc() would get called with arguments in certain
registers. While the "noinline" attribute guarantees the function to
not get inlined, it does not guarantee that all arguments arrive in the
assumed registers: gcc's interprocedural analysis can result in clone
functions to be created where some of the incoming arguments (commonly
when they have constant values) get replaced by putting in place the
respective values inside the clone.

I'm not sure to understand here. If the function is marked as noinlined,
that would mean the arguments will be passed with the ARM in the
register with the ARM calling convention (i.e r0 for argument 0...). Why
GCC would try to create a clone of this function?

The compiler is free to do so as long as the language specification isn't
being violated.

Thanks for the explanation.

The alternative of adding __attribute__((optimize("-fno-ipa-cp")))
to the function definition would likely not work with all supported
compiler versions.

The function was first introduced in arch/arm/psci.c (it's a copy of the
Linux one in arch/arm/kernel/psci.c).

Does it mean that Linux code is buggy too?


This function is duplicate in 3 different places in Xen:
        - arch/arm/psci.c
        - arch/platforms/exynos5.c
        - arch/platforms/seattle.c

So all those functions should be fixed. I think it's time to introduce a
global SMC function...

Okay, I got the build failure only in this one place. But if and when
the compiler choses to do such transformations is entirely up to it,
so yes, if there are multiple instances likely they all would need

BTW,  named register is a GNU extension and not supported by clang. Can
you avoid to use them? Maybe by writing the function in assembly. So we
are safe against any compiler optimization.

I think Jan's patch (or something like it either applied to all three
sites or a new consolidated single site) is good enough for now, given
we are in a freeze.

If you want to rewrite in asm to support clang then that can be done as
a follow up.

I find pointless to do a follow-up later if we decide to go to a consolidated single site.

The freeze period doesn't mean we need to do something that we know won't work on different compiler. Mainly when it will take the same time to write the code (~2 lines of assembly).

I'm fine to write those 2 lines of assembly if this is a matter of lake of time.


Julien Grall

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