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

[Marc Zyngier <maz@kernel.org>]

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.

> > 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.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.