Re: [XEN PATCH][for-4.19 v4 1/8] xen/include: add macro ISOLATE_LOW_BIT

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.11.2023 11:02, Nicola Vetrini wrote:
> On 2023-11-16 09:26, Jan Beulich wrote:
>> On 31.10.2023 11:20, Jan Beulich wrote:
>>> On 31.10.2023 11:03, Nicola Vetrini wrote:
>>>> On 2023-10-31 09:28, Nicola Vetrini wrote:
>>>>> On 2023-10-31 08:43, Jan Beulich wrote:
>>>>>> On 30.10.2023 23:44, Stefano Stabellini wrote:
>>>>>>> On Mon, 30 Oct 2023, Jan Beulich wrote:
>>>>>>>> On 27.10.2023 15:34, Nicola Vetrini wrote:
>>>>>>>>> --- a/xen/include/xen/macros.h
>>>>>>>>> +++ b/xen/include/xen/macros.h
>>>>>>>>> @@ -8,8 +8,14 @@
>>>>>>>>>  #define DIV_ROUND(n, d) (((n) + (d) / 2) / (d))
>>>>>>>>>  #define DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
>>>>>>>>>
>>>>>>>>> -#define MASK_EXTR(v, m) (((v) & (m)) / ((m) & -(m)))
>>>>>>>>> -#define MASK_INSR(v, m) (((v) * ((m) & -(m))) & (m))
>>>>>>>>> +/*
>>>>>>>>> + * Given an unsigned integer argument, expands to a mask where
>>>>>>>>> just the least
>>>>>>>>> + * significant nonzero bit of the argument is set, or 0 if no 
>>>>>>>>> bits
>>>>>>>>> are set.
>>>>>>>>> + */
>>>>>>>>> +#define ISOLATE_LOW_BIT(x) ((x) & -(x))
>>>>>>>>
>>>>>>>> Not even considering future Misra changes (which aiui may require
>>>>>>>> that
>>>>>>>> anyway), this generalization of the macro imo demands that its
>>>>>>>> argument
>>>>>>>> now be evaluated only once.
>>>>>>>
>>>>>>> Fur sure that would be an improvement, but I don't see a trivial 
>>>>>>> way
>>>>>>> to
>>>>>>> do it and this issue is also present today before the patch.
>>>>>>
>>>>>> This was an issue here for MASK_EXTR() and MASK_INSR(), yes, but 
>>>>>> the
>>>>>> new
>>>>>> macro has wider use, and there was no issue elsewhere so far.
>>>>>>
>>>>>>> I think it
>>>>>>> would be better to avoid scope-creeping this patch as we are 
>>>>>>> already
>>>>>>> at
>>>>>>> v4 for something that was expected to be a trivial mechanical 
>>>>>>> change.
>>>>>>> I
>>>>>>> would rather review the fix as a separate patch, maybe sent by you 
>>>>>>> as
>>>>>>> you probably have a specific implementation in mind?
>>>>>>
>>>>>> #define ISOLATE_LOW_BIT(x) ({ \
>>>>>>     typeof(x) x_ = (x); \
>>>>>>     x_ & -x_; \
>>>>>> })
>>>>>>
>>>>>> Hard to see the scope creep here. What I would consider scope creep 
>>>>>> I
>>>>>> specifically didn't even ask for: I'd like this macro to be
>>>>>> overridable
>>>>>> by an arch. Specifically (see my earlier naming hint) I'd like to 
>>>>>> use
>>>>>> x86's BMI insn BLSI in the context of "x86: allow Kconfig control 
>>>>>> over
>>>>>> psABI level", when ABI v2 or higher is in use.
>>>>>
>>>>> I appreciate you suggesting an implementation; I'll send a v5
>>>>> incorporating it.
>>>>
>>>> There's an issue with this approach, though: since the macro is used
>>>> indirectly
>>>> in expressions that are e.g. case labels or array sizes, the build 
>>>> fails
>>>> (see [1] for instance).
>>>> Perhaps it's best to leave it as is?
>>>
>>> Hmm. I'm afraid it's not an option to "leave as is", not the least 
>>> because
>>> - as said - I'm under the impression that another Misra rule requires
>>> macro arguments to be evaluated exactly once. Best I can think of 
>>> right
>>> away is to have a macro for limited use (to address such build issues)
>>> plus an inline function (for general use). But yes, maybe that then 
>>> indeed
>>> needs to be a 2nd step.
>>
>> While I've committed this patch (hoping that I got the necessary 
>> context
>> adjustment right for the 
>> automation/eclair_analysis/ECLAIR/deviations.ecl
>> change), I'd like to come back to this before going further with users 
>> of
>> the new macro: I still think we ought to try to get to the single
>> evaluation wherever possible. The macro would then be used only in 
>> cases
>> where the alternative construct (perhaps an isolate_lsb() macro, living
>> perhaps in xen/bitops.h) cannot be used. ISOLATE_LSB() would then want 
>> to
>> gain a comment directing people to the "better" sibling. Thoughts?
> 
> Having the users in place would help me estimate the remaining work that 
> needs to be done on this rule and see if my local counts match up with 
> the counts in staging.

By "having the users in place", you mean you want other patches in this
and the dependent series to be committed as-is (except for the name
change)? That's what I'd like to avoid, as it would mean touching all
those use sites again where the proposed isolate_lsb() could be used
instead. I'd rather see all use sites be put into their final shape
right away.

Jan