On Fri, Apr 11, 2014 at 06:16:24PM +0800, Lu Xiangyu wrote: > From: Xiangyu Lu <[email protected]> > > In big-endian systems, "%1" get the most significant part of the value, cause > the instruction to get the wrong result. > > When viewing ftrace record in big-endian ARM systems, we found that > the timestamp errors: > > swapper-0 [001] 1325.970000: 0:120:R ==> [001] 16:120:R events/1 > events/1-16 [001] 1325.970000: 16:120:S ==> [001] 0:120:R swapper > swapper-0 [000] 1325.1000000: 0:120:R + [000] 15:120:R events/0 > swapper-0 [000] 1325.1000000: 0:120:R ==> [000] 15:120:R events/0 > swapper-0 [000] 1326.030000: 0:120:R + [000] 1150:120:R sshd > swapper-0 [000] 1326.030000: 0:120:R ==> [000] 1150:120:R sshd > > When viewed ftrace records, it will call the do_div(n, base) function, which > achieved arch/arm/include/asm/div64.h in. When n = 10000000, base = 1000000, > in > do_div(n, base) will execute "umull %Q0, %R0, %1, %Q2". > > Cc: <[email protected]> # 2.6.20+ > Signed-off-by: Alex Wu <[email protected]> > Signed-off-by: Xiangyu Lu <[email protected]> > --- > arch/arm/include/asm/div64.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h > index 191ada6..662c7bd 100644 > --- a/arch/arm/include/asm/div64.h > +++ b/arch/arm/include/asm/div64.h > @@ -156,7 +156,7 @@ > /* Select the best insn combination to perform the */ \ > /* actual __m * __n / (__p << 64) operation. */ \ > if (!__c) { \ > - asm ( "umull %Q0, %R0, %1, %Q2\n\t" \ > + asm ( "umull %Q0, %R0, %Q1, %Q2\n\t" \
This looks plausible: these if() clauses are all concerned with multiplying the low parts of __m and __n together, and this seems to be the only 64-bit asm operand reference where Q or R is suspiciously missing: so it looks likely that "Q" is required here for consistency. My understanding of the details of this code are limited: do you have a simple test case to demonstrate the error and the fix? It should be sufficient to find two values a and b, where do_div(a,b) demonstrates the bug. Cheers ---Dave -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
