[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 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
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