Re: [PATCH v8 16/29] arm64: Share arm64 headers with s390

[Steffen Eiden <seiden@linux.ibm.com>]

On Wed, Sep 30, 2026 at 08:55:49AM +0100, Marc Zyngier wrote:
> On Wed, 30 Sep 2026 08:28:03 +0100,
> Steffen Eiden <seiden@linux.ibm.com> wrote:
> > 
> > On Tue, Sep 29, 2026 at 06:00:12PM +0100, Catalin Marinas wrote:
> > > On Tue, Sep 29, 2026 at 06:19:20AM +0200, Andreas Grapentin wrote:
> > > > On Sep 28 26, Steffen Eiden wrote:
> > > > > On Mon, Sep 28, 2026 at 05:07:52PM +0100, Catalin Marinas wrote:
> > > > > > On Fri, Sep 18, 2026 at 03:30:53PM +0200, Steffen Eiden wrote:
> > > > > > > +# Enable all code shared to s390
> > > > > > > +KBUILD_CFLAGS	+= -DARM64_S390_COMMON
> > > > > > > +KBUILD_AFLAGS	+= -DARM64_S390_COMMON
> > > > > > > +KBUILD_CPPFLAGS	+= -DARM64_S390_COMMON
> > > > > > 
> > > > > > Do we actually need these defines? They seem only to be used as markers
> > > > > > for the awk scripts to extract the definitions. Why do we need the C
> > > > > > preprocessor involved at all? Could we not just have comment markers:
> > > > > > 
> > > > > > /* ARM64_S390_COMMON_BEGIN */
> > > > > > ...
> > > > > > /* ARM64_S390_COMMON_END */
> > > > > >
> > > > > No technically we do not need those. They could be useful if we find out
> > > > > that AWK is the wrong tool and move to a C Preprocessor + diff based
> > > > > approach. 
> > > > > 
> > > > > if the ifdev is not closed the compiler will complain, but an 'arm did
> > > > > not destroy us' verifiaction tool ( I will send one soonish) could do
> > > > > the same.
> > > > 
> > > > iirc the last time we discussed this we didn't think that load-bearing
> > > > comments were the right tool here. We also briefly floated the idea of
> > > > using a #pragma region based approach, but in the end we went with the
> > > > #ifdef preprocessor directives instead, as the least invasive
> > > > non-comment marker that was available.
> > > 
> > > The downside is that the macro affects the preprocessed code. We need to
> > > ensure they don't leak in uapi headers for example. Not a fan of this
> > > approach but I don't have a better suggestion either. At least we could
> > > write them as:
> > > 
> > > #if ARM64_S390_COMMON == 1
> > 
> > This is a great idea and an improvement. Thanks.
> > 
> > @Marc: Shall I sent a v9 of this series or do you want to pick v8 and I
> > send this as an improvement once picked?
> 
> Please send a v9. That's invasive enough that it warrants it.

Will send one ASAP.

> 
> > > It would also be useful for the awk scripts look for the paired #endif
> > > rather than relying on the comment at the end of the line. Otherwise
> > > it's not really different from just sticking to comments.
> > > 
> > 
> > Are you suggesting that the AWK scripts should track the #if% and #endif
> > in the code and warn or reject if the marked endif for the shared code
> > does not match pairing with the starting #if? That makes sense yes. I'll
> > investigate and would add this as an improvement later on.
> 
> I think the suggestion is to teach the script about the #if/#endif
> nesting, and therefore to not require the trailing comment on #endif
> (i.e. the script acts as a normal cpp).
> 
> It should only be a matter of counting the #{if,ifdef}s and #endif
> past the ARM64_S390_COMMON guard.
>

Yes, countin is what I had in mind. I will deliver this feature (or fix?)
as an improvement later on. At this point we may actually use the cpp to
do that for us. This requires some investigation from my side. IMO the
current implementation is a good first step we can use as a starting
point for further improvements.

	Steffen