From mboxrd@z Thu Jan 1 00:00:00 1970 From: gangchen@rdamicro.com (=?utf-8?B?6ZmI5YiaKEdhbmdjaGVuKQ==?=) Date: Wed, 30 Mar 2016 03:27:01 +0000 Subject: [PATCH 1/1] ARM : missing corrupted reg in __do_div_asm In-Reply-To: <20160329105637.GD3701@e103592.cambridge.arm.com> References: <1459138743-10477-1-git-send-email-chengang.beijing@gmail.com> <3792990.eCI4tPEEyD@wuerfel> <20160329102605.GC3701@e103592.cambridge.arm.com> <20160329103418.GX19428@n2100.arm.linux.org.uk> <20160329105637.GD3701@e103592.cambridge.arm.com> Message-ID: <56FB4784.1060105@rdamicro.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 03/29/2016 06:56 PM, Dave Martin wrote: > On Tue, Mar 29, 2016 at 11:34:18AM +0100, Russell King - ARM Linux wrote: >> On Tue, Mar 29, 2016 at 11:26:05AM +0100, Dave Martin wrote: >>> On Tue, Mar 29, 2016 at 12:19:49PM +0200, Arnd Bergmann wrote: >>>> On Monday 28 March 2016 12:19:03 Chen Gang wrote: >>>>> __xl(R0 in little endian system, or R1 in big endian system) is corrupted >>>>> after calling __do_div64 and compiler is not informed about this in >>>>> macro __do_div_asm. If n is used again afterwards, __xl won't be >>>>> reloaded and n will contain incorrect value. >>>>> >>>>> Signed-off-by: Chen Gang >>>>> Signed-off-by: Chen Gang >>>>> --- >>>> How did you find this? Did you run into this problem on a live system >>>> or see it through inspection? >>>> >>>>> arch/arm/include/asm/div64.h | 6 ++++-- >>>>> 1 file changed, 4 insertions(+), 2 deletions(-) >>>>> >>>>> diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h >>>>> index e1f0776..1a6e91a 100644 >>>>> --- a/arch/arm/include/asm/div64.h >>>>> +++ b/arch/arm/include/asm/div64.h >>>>> @@ -35,12 +35,14 @@ static inline uint32_t __div64_32(uint64_t *n, uint32_t base) >>>>> register unsigned long long __n asm("r0") = *n; >>>>> register unsigned long long __res asm("r2"); >>>>> register unsigned int __rem asm(__xh); >>>>> + register unsigned int __clobber asm(__xl); >>>>> asm( __asmeq("%0", __xh) >>>>> __asmeq("%1", "r2") >>>>> + __asmeq("%3", "r0") >>>>> + __asmeq("%4", "r4") >>>>> __asmeq("%2", "r0") >>>>> - __asmeq("%3", "r4") >>>>> "bl __do_div64" >>>>> - : "=r" (__rem), "=r" (__res) >>>>> + : "=r" (__rem), "=r" (__res), "=r" (__clobber) >>>>> : "r" (__n), "r" (__base) >>>>> : "ip", "lr", "cc"); >>>>> *n = __res; >>>> Doesn't the clobber normally go in the third line along with >>>> "ip" and "lr"? >>> Since __xl is not used for any real argument to the asm, I think >>> we can just add __xl to the clobber list directly, without needing >>> to introduce an extra register variable ... no? >> No, you can't. The clobber list is not allowed to specify registers >> that may be used for input or output operands, and since __xl may be >> r0, and __n _is_ r0, you can't specify r0 in the clobber list. > Hmm, you're right -- in which case the change looks reasonable. > > I wonder whether the following would be cleaner than having these > aliased arguments: > > asm( /* ... */ > "bl __do_div64" > : "+r" (__n), "=r" (__res) > : "r" (__base) > : "ip", "lr", "cc"); > *n = __res; > return __n >> 32; > > (providing that GCC doesn't make a mess of the "easy" shift). I tried your proposal. It didn't make any difference: this is inline function and gcc just ignores your trick. > > Cheers > ---Dave