[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [XEN PATCH][for-4.19 v4 1/8] xen/include: add macro ISOLATE_LOW_BIT
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? Jan
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