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Message-ID: <20200124032726.GQ30412@brightrain.aerifal.cx> Date: Thu, 23 Jan 2020 22:27:26 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: arm __a_barrier_v6 register value should be zero? 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@...c.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
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