Re: [XEN PATCH v2 00/10] address violations of MISRA C:2012 Directive 4.10

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.10.2023 01:26, Stefano Stabellini wrote:
> On Fri, 20 Oct 2023, Jan Beulich wrote:
>> On 19.10.2023 18:19, Stefano Stabellini wrote:
>>> On Thu, 19 Oct 2023, Jan Beulich wrote:
>>>> On 19.10.2023 02:44, Stefano Stabellini wrote:
>>>>> On Wed, 18 Oct 2023, Jan Beulich wrote:
>>>>>> On 18.10.2023 02:48, Stefano Stabellini wrote:
>>>>>>> On Mon, 16 Oct 2023, Jan Beulich wrote:
>>>>>>>> On 29.09.2023 00:24, Stefano Stabellini wrote:
>>>>>>>>> If it is not a MISRA requirement, then I think we should go for the 
>>>>>>>>> path
>>>>>>>>> of least resistance and try to make the smallest amount of changes
>>>>>>>>> overall, which seems to be:
>>>>>>>>
>>>>>>>> ... "least resistance" won't gain us much, as hardly any guards don't
>>>>>>>> start with an underscore.
>>>>>>>>
>>>>>>>>> - for xen/include/blah.h, __BLAH_H__
>>>>>>>>> - for xen/arch/arm/asm/include/blah.h, __ASM_ARM_BLAH_H__
>>>>>>>>> - for xen/arch/x86/asm/include/blah.h, it is far less consistent, 
>>>>>>>>> maybe __ASM_X86_BLAH_H__ ?
>>>>>>>>
>>>>>>>> There are no headers in xen/include/. For (e.g.) xen/include/xen/ we
>>>>>>>> may go with XEN_BLAH_H; whether ASM prefixes are needed I'm not sure;
>>>>>>>> we could go with just ARM_BLAH_H and X86_BLAH_H?
>>>>>>>>
>>>>>>>> The primary question though is (imo) how to deal with private headers,
>>>>>>>> such that the risk of name collisions is as small as possible.
>>>>>>>
>>>>>>> Looking at concrete examples under xen/include/xen:
>>>>>>> xen/include/xen/mm.h __XEN_MM_H__
>>>>>>> xen/include/xen/dm.h __XEN_DM_H__
>>>>>>> xen/include/xen/hypfs.h __XEN_HYPFS_H__
>>>>>>>
>>>>>>> So I think we should do for consistency:
>>>>>>> xen/include/xen/blah.h __XEN_BLAH_H__
>>>>>>>
>>>>>>> Even if we know the leading underscore are undesirable, in this case I
>>>>>>> would prefer consistency.
>>>>>>
>>>>>> I'm kind of okay with that. FTAOD - here and below you mean to make this
>>>>>> one explicit first exception from the "no new leading underscores" goal,
>>>>>> for newly added headers?
>>>>>
>>>>> Yes. The reason is for consistency with the existing header files.
>>>>>
>>>>>
>>>>>>> On the other hand looking at ARM examples:
>>>>>>> xen/arch/arm/include/asm/traps.h __ASM_ARM_TRAPS__
>>>>>>> xen/arch/arm/include/asm/time.h __ARM_TIME_H__
>>>>>>> xen/arch/arm/include/asm/sysregs.h __ASM_ARM_SYSREGS_H
>>>>>>> xen/arch/arm/include/asm/io.h _ASM_IO_H
>>>>>>>
>>>>>>> And also looking at x86 examples:
>>>>>>> xen/arch/x86/include/asm/paging.h _XEN_PAGING_H
>>>>>>> xen/arch/x86/include/asm/p2m.h _XEN_ASM_X86_P2M_H
>>>>>>> xen/arch/x86/include/asm/io.h _ASM_IO_H
>>>>>>>
>>>>>>> Thet are very inconsistent.
>>>>>>>
>>>>>>>
>>>>>>> So for ARM and X86 headers I think we are free to pick anything we want,
>>>>>>> including your suggested ARM_BLAH_H and X86_BLAH_H. Those are fine by
>>>>>>> me.
>>>>>>
>>>>>> To be honest, I'd prefer a global underlying pattern, i.e. if common
>>>>>> headers are "fine" to use leading underscores for guards, arch header
>>>>>> should, too.
>>>>>
>>>>> I am OK with that too. We could go with:
>>>>> __ASM_ARM_BLAH_H__
>>>>> __ASM_X86_BLAH_H__
>>>>>
>>>>> I used "ASM" to make it easier to differentiate with the private headers
>>>>> below. Also the version without "ASM" would work but it would only
>>>>> differ with the private headers in terms of leading underscores. I
>>>>> thought that also having "ASM" would help readability and help avoid
>>>>> confusion.
>>>>>
>>>>>
>>>>>>> For private headers such as:
>>>>>>> xen/arch/arm/vuart.h __ARCH_ARM_VUART_H__
>>>>>>> xen/arch/arm/decode.h __ARCH_ARM_DECODE_H_
>>>>>>> xen/arch/x86/mm/p2m.h __ARCH_MM_P2M_H__
>>>>>>> xen/arch/x86/hvm/viridian/private.h X86_HVM_VIRIDIAN_PRIVATE_H
>>>>>>>
>>>>>>> More similar but still inconsistent. I would go with ARCH_ARM_BLAH_H and
>>>>>>> ARCH_X86_BLAH_H for new headers.
>>>>>>
>>>>>> I'm afraid I don't like this, as deeper paths would lead to unwieldy
>>>>>> guard names. If we continue to use double-underscore prefixed names
>>>>>> in common and arch headers, why don't we demand no leading underscores
>>>>>> and no path-derived prefixes in private headers? That'll avoid any
>>>>>> collisions between the two groups.
>>>>>
>>>>> OK, so for private headers:
>>>>>
>>>>> ARM_BLAH_H
>>>>> X86_BLAH_H
>>>>>
>>>>> What that work for you?
>>>>
>>>> What are the ARM_ and X86_ prefixes supposed to indicate here? Or to ask
>>>> differently, how would you see e.g. common/decompress.h's guard named?
>>>
>>> I meant that:
>>>
>>> xen/arch/arm/blah.h would use ARM_BLAH_H
>>> xen/arch/x86/blah.h would use X86_BLAH_H
>>>
>>> You have a good question on something like xen/common/decompress.h and
>>> xen/common/event_channel.h.  What do you think about:
>>>
>>> COMMON_BLAH_H, so specifically COMMON_DECOMPRESS_H
>>>
>>> otherwise:
>>>
>>> XEN_BLAH_H, so specifically XEN_DECOMPRESS_H
>>>
>>> I prefer COMMON_BLAH_H but I think both versions are OK.
>>
>> IOW you disagree with my earlier "... and no path-derived prefixes",
>> and you prefer e.g. DRIVERS_PASSTHROUGH_VTD_DMAR_H as a consequence?
>> FTAOD my earlier suggestion was simply based on the observation that
>> the deeper the location of a header in the tree, the more unwieldy
>> its guard name would end up being if path prefixes were to be used.
> 
> I don't have a strong opinion on "path-derived prefixes". I prefer
> consistency and easy-to-figure-out guidelines over shortness.
> 
> The advantage of a path-derived prefix is that it is trivial to figure
> out at first glance. If we can come up with another system that is also
> easy then fine. Do you have a suggestion? If so, sorry I missed it.

Well, I kind of implicitly suggested "no path derived prefixes for private
headers", albeit realizing that there's a chance then of guards colliding.
I can't think of a good scheme which would fit all goals (no collisions,
uniformity, and not unduly long).

Jan