Re: [PATCH v2 11/18] arm64: make mrs_s and msr_s macros work with LTO

Yury Norov
2017-11-16T21:29:42Z

On Thu, Nov 16, 2017 at 01:55:31PM +0000, Robin Murphy wrote:
> On 16/11/17 13:07, Yury Norov wrote:
> > On Thu, Nov 16, 2017 at 11:54:33AM +0000, Will Deacon wrote:
> > > On Wed, Nov 15, 2017 at 01:34:21PM -0800, Sami Tolvanen wrote:
> > > > From: Alex Matveev <alxmtvv@gmail.com>
> > > >
> > > > Use UNDEFINE_MRS_S and UNDEFINE_MSR_S to define corresponding macros
> > > > in-place and workaround gcc and clang limitations on redefining macros
> > > > across different assembler blocks.
> > >
> > > What limitations? Can you elaborate please? Is this a fix?
> >
> > Hi Will,
> >
> > Regarding GCC.
> >
> > When it joins preprocessed source files into single asm file,
> > mrs_s/msr_s becomes either not declared or declared multiple times.
> >
> > ./ccuFb68h.s:33120: Error: Macro `mrs_s' was already defined
> > ./ccuFb68h.s:33124: Error: Macro `msr_s' was already defined
> >
> > I'm not sure that GCC works correctly in this case, and I sent the
> > email to Linaro toolchain group to clarify it. See below.
> >
> > Yury
> >
> > [...]
> >
> > Links:
> > My unfinished branch:
> > https://github.com/norov/linux/tree/lto
> > Andi Kleen tree:
> > https://github.com/andikleen/linux-misc/tree/lto-411-1
> > Sami Tolvanen's recent work for clang:
> > https://lkml.org/lkml/2017/11/3/606
> >
> > Question we have for now:
> > There's mrs_s/msr_s macro that doesn't work with LTO - linker
> > complains very loudly that macro is either not declared, or declared
> > multiple times. (To reproduce - try to build my kernel branch w/o last
> > patch).
> >
> > The same (?) problem is observed with clang, and people there
> > considered it as feature, not a bug.
> >
> > https://bugs.llvm.org/show_bug.cgi?id=19749
> >
> > We have the fix for both clang and gcc, but it looks hacky. Maybe it
> > worth to fix mrs/msr issue on toolchain side?
>
> Given that this whole mrs_s infrastructure is a workaround for older
> assemblers which don't support the "S<op0>_<op1>_<Cn>_<Cm>_<op2>" syntax for
> arbitrary unnamed system registers (which IIRC was a fairly late addition to
> the architecture), the only way it could be "fixed" on the toolchain side is
> by removing all those older toolchains from existence. Good luck with that
> ;)
>
> In *theory*, it might be possible to do something similar to what we do with
> CONFIG_BROKEN_GAS_INST to detect offending assemblers and only define and
> use these macros when necessary (hopefully Clang and other LTO-capable
> toolchains do accept the proper syntax), but I've no idea how invasive or
> difficult that might turn out to be.

Thank you Robin. The "S<op0>_<op1>_<Cn>_<Cm>_<op2>" is the feature I
was looking for. I'm not going to remove old toolchain from every user
machine - I believe if I do, I'll be put into jail finally. :)

But I think we should use this syntax for {read,write}_sysreg_s
and other functions if assembler supports it. The existing mrs_s/msr_s
machinery will be deprecated then, and it will be generally good.

Sami told that LTO doesn't work with old toolchains, so I think that
it would be OK to disable LTO for old assemblers that doesn't support
all necessary features. Anyway, this is not blocking issue because we
have patch that workarounds it (again).

Yury