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

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

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:
> > diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> > index 6b005c8fef70..12cbad460258 100644
> > --- a/arch/arm64/Makefile
> > +++ b/arch/arm64/Makefile
> > @@ -45,6 +45,11 @@ KBUILD_CFLAGS	+= $(CC_FLAGS_NO_FPU) \
> >  KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)
> >  KBUILD_AFLAGS	+= $(compat_vdso)
> >  
> > +# 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.

If the idfef thing is a no-go for you I can use the solely comment based
approach.

> (also the CPPFLAGS definition was enough, it gets copied into the others
> automatically)

ok

> 
> > diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> > index ab205f9db94a..1c5c4df260be 100644
> > --- a/arch/arm64/include/asm/sysreg.h
> > +++ b/arch/arm64/include/asm/sysreg.h
> > @@ -16,6 +16,8 @@
> >  
> >  #include <asm/gpr-num.h>
> >  
> > +#ifdef ARM64_S390_COMMON
> > +
> >  /*
> >   * ARMv8 ARM reserves the following encoding for system registers:
> >   * (Ref: ARMv8 ARM, Section: "System instruction class encoding overview",
> > @@ -50,6 +52,8 @@
> >  #define sys_reg_CRm(id)	(((id) >> CRm_shift) & CRm_mask)
> >  #define sys_reg_Op2(id)	(((id) >> Op2_shift) & Op2_mask)
> >  
> > +#endif /* ARM64_S390_COMMON */
> > +
> >  #ifndef CONFIG_BROKEN_GAS_INST
> >  
> >  #ifdef __ASSEMBLER__
> > @@ -123,6 +127,8 @@
> >  #define GSB_SYS_BARRIER_INSN		__SYS_BARRIER_INSN(1, 0, 12, 0, 0, 31)
> >  #define GSB_ACK_BARRIER_INSN		__SYS_BARRIER_INSN(1, 0, 12, 0, 1, 31)
> >  
> > +#ifdef ARM64_S390_COMMON
> > +
> >  /* Data cache zero operations */
> >  #define SYS_DC_ISW			sys_insn(1, 0, 7, 6, 2)
> >  #define SYS_DC_IGSW			sys_insn(1, 0, 7, 6, 4)
> > @@ -832,6 +838,8 @@
> >  #define SCTLR_ELx_A	 (BIT(1))
> >  #define SCTLR_ELx_M	 (BIT(0))
> >  
> > +#endif /* ARM64_S390_COMMON */
> > +
> >  #ifdef CONFIG_CPU_BIG_ENDIAN
> >  #define ENDIAN_SET_EL2		SCTLR_ELx_EE
> >  #else
> > @@ -866,6 +874,7 @@
> >  	 SCTLR_EL1_LSMAOE | SCTLR_EL1_nTLSMD | SCTLR_EL1_EIS   | \
> >  	 SCTLR_EL1_TSCXT  | SCTLR_EL1_EOS)
> >  
> > +#ifdef ARM64_S390_COMMON
> >  /* MAIR_ELx memory attributes (used by Linux) */
> >  #define MAIR_ATTR_DEVICE_nGnRnE		UL(0x00)
> >  #define MAIR_ATTR_DEVICE_nGnRE		UL(0x04)
> > @@ -1102,6 +1111,8 @@
> >  #define GICV5_GICR_CDNMIA_TYPE_MASK	GENMASK_ULL(31, 29)
> >  #define GICV5_GICR_CDNMIA_ID_MASK	GENMASK_ULL(23, 0)
> >  
> > +#endif /* ARM64_S390_COMMON */
> 
> I haven't checked them all but there are a few definitions in here that
> depend on arm64-specific configs: e.g. GCR depends on KASAN, TGRAN
> macros depend on page size, PA_BITS_52 influences some other values.
> 
> I think they should be outside the common definitions shared with s390.
> In addition, the awk scripts should reject any CONFIG_ (or at least
> CONFIG_ARM64_) lines in the copied files. Ideally report an error rather
> than silently masking them out.
> 

Upon Marcs request (and to keep me sane) we deliberately share more
header stuff than we actually need to
a) keep the number of shared regions low
b) have fewer prepare changes moving things to satisfy a

I do not see an issue if we have a few things shared that are only
active on ARM due to a CONFIG. 

	Steffen

> -- 
> Catalin