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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan

> [1] https://gitlab.com/xen-project/people/bugseng/xen/-/jobs/5423693947
>