On Fri, Apr 11, 2014 at 06:16:24PM +0800, Lu Xiangyu wrote:
> From: Xiangyu Lu <luxiangyu@xxxxxxxxxx>
>
> 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: <stable@xxxxxxxxxxxxxxx> # 2.6.20+
> Signed-off-by: Alex Wu <wuquanming@xxxxxxxxxx>
> Signed-off-by: Xiangyu Lu <luxiangyu@xxxxxxxxxx>
> ---
> 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
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