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Re: [PATCH] xen/mm: Remove claim that INVALID_{MFN,GFN}_INITIALIZER is for older toolchains
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 25 Feb 2026 16:35:47 +0000
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 25 Feb 2026 16:36:01 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 25/02/2026 2:34 pm, Jan Beulich wrote:
> On 25.02.2026 13:59, Andrew Cooper wrote:
>> This was never because of a bug in GCC.
>>
>> C requires that static objects are initialised with constant expressions;
>> _mfn(), as a static inline, is not and cannot be made to be.
> Of course, and I think the comments were meant differently. What wasn't
> possible
> to use (with -std=gnu99) due to the referenced bug is apparently
>
> #define INVALID_GFN ((gfn_t){ ~0UL })
>
> Now that gcc5 is our baseline, do we perhaps want to use that and do away with
> INVALID_GFN_INITIALIZER?
Oh. Yeah that's very much not what the comment suggested.
Changing like that almost works, but there's one snag. common/memory.c has
BUILD_BUG_ON(INVALID_GFN_RAW + 1);
and with the _RAW constant wanting to go, the obvious:
BUILD_BUG_ON(gfn_x(INVALID_GFN) + 1);
doesn't compile as it's no longer a constant expression.
It's not clear what to do here. I don't think we want to keep
INVALID_GFN_RAW around for just this, but nor am I completely happy
dropping the BUILD_BUG_ON() either.
Thoughts?
~Andrew
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