Re: [kvm-unit-tests PATCH 1/8] s390x: lib: Extend bitops

[Janosch Frank <frankja@linux.ibm.com>]

On 8/18/21 10:20 AM, Thomas Huth wrote:
> On 13/08/2021 13.31, Janosch Frank wrote:
>> On 8/13/21 10:32 AM, Claudio Imbrenda wrote:
>>> On Fri, 13 Aug 2021 07:36:08 +0000
>>> Janosch Frank <frankja@linux.ibm.com> wrote:
>>>
>>>> Bit setting and clearing is never bad to have.
>>>>
>>>> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
>>>> ---
>>>>   lib/s390x/asm/bitops.h | 102
>>>> +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 102
>>>> insertions(+)
>>>>
>>>> diff --git a/lib/s390x/asm/bitops.h b/lib/s390x/asm/bitops.h
>>>> index 792881ec..f5612855 100644
>>>> --- a/lib/s390x/asm/bitops.h
>>>> +++ b/lib/s390x/asm/bitops.h
>>>> @@ -17,6 +17,78 @@
>>>>   
>>>>   #define BITS_PER_LONG	64
>>>>   
>>>> +static inline unsigned long *bitops_word(unsigned long nr,
>>>> +					 const volatile unsigned
>>>> long *ptr) +{
>>>> +	unsigned long addr;
>>>> +
>>>> +	addr = (unsigned long)ptr + ((nr ^ (nr & (BITS_PER_LONG -
>>>> 1))) >> 3);
>>>> +	return (unsigned long *)addr;
>>>
>>> why not just
>>>
>>> return ptr + (nr / BITS_PER_LONG);
>>>
>>>> +}
>>>> +
>>>> +static inline unsigned long bitops_mask(unsigned long nr)
>>>> +{
>>>> +	return 1UL << (nr & (BITS_PER_LONG - 1));
>>>> +}
>>>> +
>>>> +static inline uint64_t laog(volatile unsigned long *ptr, uint64_t
>>>> mask) +{
>>>> +	uint64_t old;
>>>> +
>>>> +	/* load and or 64bit concurrent and interlocked */
>>>> +	asm volatile(
>>>> +		"	laog	%[old],%[mask],%[ptr]\n"
>>>> +		: [old] "=d" (old), [ptr] "+Q" (*ptr)
>>>> +		: [mask] "d" (mask)
>>>> +		: "memory", "cc" );
>>>> +	return old;
>>>> +}
>>>
>>> do we really need the artillery (asm) here?
>>> is there a reason why we can't do this in C?
>>
>> Those are the interlocked/atomic instructions and even though we don't
>> exactly need them right now I wanted to add them for completeness.
> 
> I think I agree with Claudio - unless we really need them, we should not 
> clog the sources with arbitrary inline assembly functions.

Alright I can trim it down

> 
>> We might be able to achieve the same via compiler functionality but this
>> is not my expertise. Maybe Thomas or David have a few pointers for me?
> 
> I'm not an expert with atomic builtins either, but what's the point of this 
> at all? Loading a value and OR-ing something into the value in one go? 
> What's that good for?

Well it's a block-concurrent interlocked-update load, or and store.
I.e. it loads the data from the ptr and copies it into [old] then ors
the mask and stores it back to the ptr address.

The instruction name "load and or" does not represent the full actions
of the instruction.

> 
>   Thomas
>