Re: [kvm-unit-tests PATCH v6 3/4] s390x: lib: add SPX and STPX instruction wrapper

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

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On 1/9/20 5:58 PM, Thomas Huth wrote:
> On 09/01/2020 17.50, Claudio Imbrenda wrote:
>> On Thu, 9 Jan 2020 17:43:55 +0100
>> Thomas Huth <thuth@redhat.com> wrote:
>>
>>> On 09/01/2020 17.16, Claudio Imbrenda wrote:
>>>> Add a wrapper for the SET PREFIX and STORE PREFIX instructions, and
>>>> use it instead of using inline assembly everywhere.
>>>>
>>>> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
>>>> ---
>>>>  lib/s390x/asm/arch_def.h | 10 ++++++++++
>>>>  s390x/intercept.c        | 33 +++++++++++++--------------------
>>>>  2 files changed, 23 insertions(+), 20 deletions(-)
>>>>
>>>> diff --git a/lib/s390x/asm/arch_def.h b/lib/s390x/asm/arch_def.h
>>>> index 1a5e3c6..465fe0f 100644
>>>> --- a/lib/s390x/asm/arch_def.h
>>>> +++ b/lib/s390x/asm/arch_def.h
>>>> @@ -284,4 +284,14 @@ static inline int servc(uint32_t command,
>>>> unsigned long sccb) return cc;
>>>>  }
>>>>  
>>>> +static inline void spx(uint32_t *new_prefix)  
>>>
>>> Looking at this a second time ... why is new_prefix a pointer? A
>>> normal value should be sufficient here, shouldn't it?
>>
>> no. if you look at the code in the same patch, intercept.c at some
>> points needs to pass "wrong" pointers to spx and stpx in order to test
>> them, so this needs to be a pointer
>>
>> the instructions themselves expect pointers (base register + offset)
> 
> Ah, you're right, that "Q" constraint always confuses me... I guess you
> could do it without pointers when using the "r" constraint, but it's
> likely better to do it the same way as stpx, so your patch should be fine.

Honestly, I'd rather have stpx return a u32 than passing a ptr.
That's how the kernel does it and is in-line with epswe/lpswe and
sctlg/lctlg which are already in the library.

Also, if possible names like set_prefix and store_prefix (or better
get_prefix) prefix would make it much more readable.

> 
>>>> +{
>>>> +	asm volatile("spx %0" : : "Q" (*new_prefix) : "memory");
>>>> +}
>>>> +
>>>> +static inline void stpx(uint32_t *current_prefix)
>>>> +{
>>>> +	asm volatile("stpx %0" : "=Q" (*current_prefix));
>>>> +}
>>>> +  
> 
> Reviewed-by: Thomas Huth <thuth@redhat.com>
> 



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