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

[Julien Grall <julien.grall@xxxxxxxxxx>]

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?
> > > Likely.
> > >
> > > > 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
> > > fixing.
> > 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.
Regards,

--
Julien Grall

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