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Re: [Xen-devel] [PATCH 4/4] xen/percpu: Make DECLARE_PER_CPU() and __DEFINE_PER_CPU() common
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 29 Jul 2019 19:03:01 +0100
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- Cc: Juergen Gross <JGross@xxxxxxxx>, StefanoStabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 29 Jul 2019 18:03:28 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 29/07/2019 14:54, Jan Beulich wrote:
> On 29.07.2019 15:23, Andrew Cooper wrote:
>> On 29/07/2019 14:00, Jan Beulich wrote:
>>> On 26.07.2019 23:08, Andrew Cooper wrote:
>>>> --- a/xen/include/xen/percpu.h
>>>> +++ b/xen/include/xen/percpu.h
>>>> @@ -3,6 +3,12 @@
>>>>
>>>> #include <asm/percpu.h>
>>>>
>>>> +#define DECLARE_PER_CPU(type, name) \
>>>> + extern __typeof__(type) per_cpu__ ## name
>>>> +
>>>> +#define __DEFINE_PER_CPU(attr, type, name) \
>>>> + attr __typeof__(type) per_cpu_ ## name
>>>> +
>>>> /*
>>>> * Separate out the type, so (int[3], foo) works.
>>>> *
>>> By moving things here you render stale the remainder of the
>>> comment in context above: No per-arch symbol name prefix is going
>>> to be possible anymore. I'm not against it, but this comment
>>> would then want adjusting. What's not immediately clear to me is
>>> whether the two-stage concatenation of an underscore each is then
>>> still necessary.
>> Yes it is still necessary. See the TSS thread for why.
> No, that thread doesn't explain it. From an initial look I think
> two-stage expansion is still necessary
It is about preventing 'name' being expanded, due to the mess with
cpumask_scratch, which requires a ## at least at the top level.
I personally think that fixing cpumask_scratch is the right way to go,
but I specifically didn't touch that so as to avoid wreaking havoc with
Juergen's core-scheduling series.
> , but it could then be
> _ ## name on the first and per_cpu ## name on the second (i.e.
> no double underscore in the middle anymore).
Hmm, probably, but...
> Of course there may
> be reasons why we actually _want_ a double underscore there.
... I don't have the effort or energy - but most importantly, time - to
rewrite Xen from scratch.
If there is a concrete reason why dropping the double underscore is
good/necessary/other, then it should be present as its own patch, not
crowbarred into an unrelated cleanup patch.
Until then, the inertia of "because its already like this" wins.
~Andrew
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