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SubjectRe: [PATCH v6] s390: Fix vDSO clock_getres()
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Hi Heiko,

On 6/9/20 3:46 PM, Heiko Carstens wrote:
> Hello Vincenzo,
>
> sorry it took such a long time to answer!
>
>> clock_getres in the vDSO library has to preserve the same behaviour
>> of posix_get_hrtimer_res().
>>
>> In particular, posix_get_hrtimer_res() does:
>> sec = 0;
>> ns = hrtimer_resolution;
>> and hrtimer_resolution depends on the enablement of the high
>> resolution timers that can happen either at compile or at run time.
>>
>> Fix the s390 vdso implementation of clock_getres keeping a copy of
>> hrtimer_resolution in vdso data and using that directly.
>>
>> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
>> Cc: Vasily Gorbik <gor@linux.ibm.com>
>> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
>> Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
>> ---
>> arch/s390/include/asm/vdso.h | 1 +
>> arch/s390/kernel/asm-offsets.c | 2 +-
>> arch/s390/kernel/time.c | 1 +
>> arch/s390/kernel/vdso64/clock_getres.S | 10 +++++-----
>> 4 files changed, 8 insertions(+), 6 deletions(-)
> ...
>
>> diff --git a/arch/s390/kernel/vdso64/clock_getres.S b/arch/s390/kernel/vdso64/clock_getres.S
>> index 081435398e0a..022b58c980db 100644
>> --- a/arch/s390/kernel/vdso64/clock_getres.S
>> +++ b/arch/s390/kernel/vdso64/clock_getres.S
>> @@ -17,12 +17,14 @@
>> .type __kernel_clock_getres,@function
>> __kernel_clock_getres:
>> CFI_STARTPROC
>> - larl %r1,4f
>> + larl %r1,3f
>> + lg %r0,0(%r1)
>> cghi %r2,__CLOCK_REALTIME_COARSE
>> je 0f
>> cghi %r2,__CLOCK_MONOTONIC_COARSE
>> je 0f
>> - larl %r1,3f
>> + larl %r1,_vdso_data
>> + l %r0,__VDSO_CLOCK_REALTIME_RES(%r1)
>
> This should be llgf for proper zero extension. The code works anyway,
> since the upper lg would clear the high order bits, however this looks
> like it works more or less by accident ;)
>
> I changed it and applied your patch. Thanks a lot!

Thank you for this and for the explanation. I must admit that this was the first
attempt to write s390 assembly. I had to go through most of the "Principles of
Operation" before I could get something meaningful going ;)

>

--
Regards,
Vincenzo

P.S. Now I can finally say that once in my life I wrote s390 assembler ;)

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