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Subject: Re: [PATCH v6] s390: Fix vDSO clock_getres()
To:     Heiko Carstens <heiko.carstens@de.ibm.com>
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
References: <20200324121027.21665-1-vincenzo.frascino@arm.com>
 <20200609144608.GB4575@osiris>
From:   Vincenzo Frascino <vincenzo.frascino@arm.com>
Message-ID: <fefd4ba9-6e00-5aba-ac95-b1fbf7e1bf13@arm.com>
Date:   Tue, 9 Jun 2020 16:12:00 +0100
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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 ;)