Re: [musl] arm __a_barrier_v6 register value should be zero?

[Rich Felker <dalias@libc.org>]

On Thu, Jan 23, 2020 at 06:09:57PM -0800, Andre McCurdy wrote:
> On Wed, Jan 22, 2020 at 8:18 PM Rich Felker <dalias@libc.org> wrote:
> > On Wed, Jan 22, 2020 at 05:30:53PM -0800, Andre McCurdy wrote:
> > > The arm1176jzfs documentation describing the armv6 CP15 Data Memory
> > > Barrier operation seems to specify the register value written to the
> > > coprocessor as "SBZ" ie Should Be Zero. See page 216 of:
> > >
> > >   http://infocenter.arm.com/help/topic/com.arm.doc.ddi0301h/DDI0301H_arm1176jzfs_r0p7_trm.pdf
> > >
> > > However the __a_barrier_v6() function which implements this in musl
> > > uses r0 as the register written to the coprocessor and doesn't
> > > initialise it. Should __a_barrier_v6() set r0 to 0 before issuing the
> > > mcr instruction? Or is it defined somewhere that this register value
> > > doesn't matter?
> >
> > __a_barrier_* have a contract not to clobber r0 -- see the calling
> > code in arch/arm/atomic_arch.h:
> >
> > static inline void a_barrier()
> > {
> >         register uintptr_t ip __asm__("ip") = __a_barrier_ptr;
> >         __asm__ __volatile__( BLX " ip" : "+r"(ip) : : "memory", "cc", "lr" );
> > }
> >
> > We could probably switch to using ip as the register if needed.
> > However looking at Linux kernel source, no effort is made to zero the
> > register used (see arch/arm/include/asm/assembler.h for the definition
> > of the smp_dmb macro) so I think it must not actually matter...
> >
> > Nice find, though. Let's see if we can get a better authoritative
> > answer on it.
> 
> OK. Another question: what's the reason for testing that the 3rd char
> of AT_PLATFORM isn't [0-9] in __set_thread_area()? Isn't just testing
> that the first two chars are "v6" enough to confirm armv6?

Indeed I suppose we should test that, though I don't seem "armv60" as
being likely now that they've mostly moved on to 64-bit for new isa
levels...

Rich