Re: [PATCH v4 2/2] powerpc/64: Option to use ELF V2 ABI for big-endian kernels

[Michal Suchánek]

On Fri, Jun 11, 2021 at 07:39:59PM +1000, Nicholas Piggin wrote:
> Provide an option to build big-endian kernels using the ELFv2 ABI. This
> works on GCC only so far, although it is rumored to work with clang
> that's not been tested yet. A new module version check ensures the
> module ELF ABI level matches the kernel build.
> 
> This can give big-endian kernels some useful advantages of the ELFv2 ABI
> (e.g., less stack usage, -mprofile-kernel, better compatibility with eBPF
> tools).
> 
> BE+ELFv2 is not officially supported by the GNU toolchain, but it works
> fine in testing and has been used by some userspace for some time (e.g.,
> Void Linux).
> 
> Tested-by: Michal Suchánek <msuchanek@xxxxxxx>
> Reviewed-by: Segher Boessenkool <segher@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
> ---
>  arch/powerpc/Kconfig                | 22 ++++++++++++++++++++++
>  arch/powerpc/Makefile               | 18 ++++++++++++------
>  arch/powerpc/boot/Makefile          |  4 +++-
>  arch/powerpc/include/asm/module.h   | 24 ++++++++++++++++++++++++
>  arch/powerpc/kernel/vdso64/Makefile | 13 +++++++++++++
>  drivers/crypto/vmx/Makefile         |  8 ++++++--
>  drivers/crypto/vmx/ppc-xlate.pl     | 10 ++++++----
>  7 files changed, 86 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 088dd2afcfe4..093f973a28b9 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -163,6 +163,7 @@ config PPC
>  	select ARCH_WEAK_RELEASE_ACQUIRE
>  	select BINFMT_ELF
>  	select BUILDTIME_TABLE_SORT
> +	select PPC64_BUILD_ELF_V2_ABI		if PPC64 && CPU_LITTLE_ENDIAN
>  	select CLONE_BACKWARDS
>  	select DCACHE_WORD_ACCESS		if PPC64 && CPU_LITTLE_ENDIAN
>  	select DMA_OPS_BYPASS			if PPC64
> @@ -561,6 +562,27 @@ config KEXEC_FILE
>  config ARCH_HAS_KEXEC_PURGATORY
>  	def_bool KEXEC_FILE
>  
> +config PPC64_BUILD_ELF_V2_ABI
> +	bool
> +
> +config PPC64_BUILD_BIG_ENDIAN_ELF_V2_ABI
> +	bool "Build big-endian kernel using ELF ABI V2 (EXPERIMENTAL)"
> +	depends on PPC64 && CPU_BIG_ENDIAN && EXPERT
> +	depends on CC_IS_GCC && LD_VERSION >= 22400
> +	default n
> +	select PPC64_BUILD_ELF_V2_ABI
> +	help
> +	  This builds the kernel image using the "Power Architecture 64-Bit ELF
> +	  V2 ABI Specification", which has a reduced stack overhead and faster
> +	  function calls. This internal kernel ABI option does not affect
> +          userspace compatibility.
> +
> +	  The V2 ABI is standard for 64-bit little-endian, but for big-endian
> +	  it is less well tested by kernel and toolchain. However some distros
> +	  build userspace this way, and it can produce a functioning kernel.
> +
> +	  This requires GCC and binutils 2.24 or newer.
> +
>  config RELOCATABLE
>  	bool "Build a relocatable kernel"
>  	depends on PPC64 || (FLATMEM && (44x || FSL_BOOKE))
> diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
> index 3212d076ac6a..b90b5cb799aa 100644
> --- a/arch/powerpc/Makefile
> +++ b/arch/powerpc/Makefile
> @@ -91,10 +91,14 @@ endif
>  
>  ifdef CONFIG_PPC64
>  ifndef CONFIG_CC_IS_CLANG
> -cflags-$(CONFIG_CPU_BIG_ENDIAN)		+= $(call cc-option,-mabi=elfv1)
> -cflags-$(CONFIG_CPU_BIG_ENDIAN)		+= $(call cc-option,-mcall-aixdesc)
> -aflags-$(CONFIG_CPU_BIG_ENDIAN)		+= $(call cc-option,-mabi=elfv1)
> -aflags-$(CONFIG_CPU_LITTLE_ENDIAN)	+= -mabi=elfv2
> +ifdef CONFIG_PPC64_BUILD_ELF_V2_ABI
> +cflags-y				+= $(call cc-option,-mabi=elfv2)
> +aflags-y				+= $(call cc-option,-mabi=elfv2)
> +else
> +cflags-y				+= $(call cc-option,-mabi=elfv1)
> +cflags-y				+= $(call cc-option,-mcall-aixdesc)
> +aflags-y				+= $(call cc-option,-mabi=elfv1)
> +endif
>  endif
>  endif
>  
> @@ -142,15 +146,17 @@ endif
>  
>  CFLAGS-$(CONFIG_PPC64)	:= $(call cc-option,-mtraceback=no)
>  ifndef CONFIG_CC_IS_CLANG
> -ifdef CONFIG_CPU_LITTLE_ENDIAN
> -CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mabi=elfv2,$(call cc-option,-mcall-aixdesc))
> +ifdef CONFIG_PPC64_BUILD_ELF_V2_ABI
> +CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mabi=elfv2)
>  AFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mabi=elfv2)
>  else
> +# Keep these in synch with arch/powerpc/kernel/vdso64/Makefile
>  CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mabi=elfv1)
>  CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mcall-aixdesc)
>  AFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mabi=elfv1)
>  endif
>  endif
> +
>  CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mcmodel=medium,$(call cc-option,-mminimal-toc))
>  CFLAGS-$(CONFIG_PPC64)	+= $(call cc-option,-mno-pointers-to-nested-functions)
>  
> diff --git a/arch/powerpc/boot/Makefile b/arch/powerpc/boot/Makefile
> index 2b8da923ceca..be84a72f8258 100644
> --- a/arch/powerpc/boot/Makefile
> +++ b/arch/powerpc/boot/Makefile
> @@ -40,6 +40,9 @@ BOOTCFLAGS    := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
>  
>  ifdef CONFIG_PPC64_BOOT_WRAPPER
>  BOOTCFLAGS	+= -m64
> +ifdef CONFIG_PPC64_BUILD_ELF_V2_ABI
> +BOOTCFLAGS	+= $(call cc-option,-mabi=elfv2)
> +endif
>  else
>  BOOTCFLAGS	+= -m32
>  endif
> @@ -50,7 +53,6 @@ ifdef CONFIG_CPU_BIG_ENDIAN
>  BOOTCFLAGS	+= -mbig-endian
>  else
>  BOOTCFLAGS	+= -mlittle-endian
> -BOOTCFLAGS	+= $(call cc-option,-mabi=elfv2)
>  endif
>  
>  BOOTAFLAGS	:= -D__ASSEMBLY__ $(BOOTCFLAGS) -nostdinc
> diff --git a/arch/powerpc/include/asm/module.h b/arch/powerpc/include/asm/module.h
> index 857d9ff24295..043e11068ff4 100644
> --- a/arch/powerpc/include/asm/module.h
> +++ b/arch/powerpc/include/asm/module.h
> @@ -52,6 +52,30 @@ struct mod_arch_specific {
>  	unsigned int num_bugs;
>  };
>  
> +/*
> + * Check kernel module ELF header architecture specific compatibility.
> + */
> +static inline bool elf_check_module_arch(Elf_Ehdr *hdr)
> +{
> +	if (!elf_check_arch(hdr))
> +		return false;
> +
> +	if (IS_ENABLED(CONFIG_PPC64)) {
> +		unsigned long abi_level = hdr->e_flags & 0x3;
> +
> +		if (IS_ENABLED(CONFIG_PPC64_BUILD_ELF_V2_ABI)) {
> +			if (abi_level != 2)
> +				return false;
> +		} else {
> +			if (abi_level >= 2)
> +				return false;
> +		}
> +	}
> +
> +	return true;
> +}
> +#define elf_check_module_arch elf_check_module_arch
> +
>  /*
>   * Select ELF headers.
>   * Make empty section for module_frob_arch_sections to expand.
Shouldn't this part go to the second patch?

Thanks

Michal
> diff --git a/arch/powerpc/kernel/vdso64/Makefile b/arch/powerpc/kernel/vdso64/Makefile
> index 2813e3f98db6..d783c07e558f 100644
> --- a/arch/powerpc/kernel/vdso64/Makefile
> +++ b/arch/powerpc/kernel/vdso64/Makefile
> @@ -25,6 +25,19 @@ KCOV_INSTRUMENT := n
>  UBSAN_SANITIZE := n
>  KASAN_SANITIZE := n
>  
> +# Always build vdso64 with ELFv1 ABI for BE kernels
> +ifdef CONFIG_PPC64_BUILD_ELF_V2_ABI
> +ifdef CONFIG_CPU_BIG_ENDIAN
> +KBUILD_CFLAGS := $(filter-out -mabi=elfv2,$(KBUILD_CFLAGS))
> +KBUILD_AFLAGS := $(filter-out -mabi=elfv2,$(KBUILD_AFLAGS))
> +
> +# These are derived from arch/powerpc/Makefile
> +KBUILD_CFLAGS += $(call cc-option,-mabi=elfv1)
> +KBUILD_CFLAGS += $(call cc-option,-mcall-aixdesc)
> +KBUILD_AFLAGS += $(call cc-option,-mabi=elfv1)
> +endif
> +endif
> +
>  ccflags-y := -shared -fno-common -fno-builtin -nostdlib \
>  	-Wl,-soname=linux-vdso64.so.1 -Wl,--hash-style=both
>  asflags-y := -D__VDSO64__ -s
> diff --git a/drivers/crypto/vmx/Makefile b/drivers/crypto/vmx/Makefile
> index 709670d2b553..d9ccf9fc3483 100644
> --- a/drivers/crypto/vmx/Makefile
> +++ b/drivers/crypto/vmx/Makefile
> @@ -5,18 +5,22 @@ vmx-crypto-objs := vmx.o aesp8-ppc.o ghashp8-ppc.o aes.o aes_cbc.o aes_ctr.o aes
>  ifeq ($(CONFIG_CPU_LITTLE_ENDIAN),y)
>  override flavour := linux-ppc64le
>  else
> +ifdef CONFIG_PPC64_BUILD_ELF_V2_ABI
> +override flavour := linux-ppc64-elfv2
> +else
>  override flavour := linux-ppc64
>  endif
> +endif
>  
>  quiet_cmd_perl = PERL $@
>        cmd_perl = $(PERL) $(<) $(flavour) > $(@)
>  
>  targets += aesp8-ppc.S ghashp8-ppc.S
>  
> -$(obj)/aesp8-ppc.S: $(src)/aesp8-ppc.pl FORCE
> +$(obj)/aesp8-ppc.S: $(src)/aesp8-ppc.pl $(src)/ppc-xlate.pl FORCE
>  	$(call if_changed,perl)
>    
> -$(obj)/ghashp8-ppc.S: $(src)/ghashp8-ppc.pl FORCE
> +$(obj)/ghashp8-ppc.S: $(src)/ghashp8-ppc.pl $(src)/ppc-xlate.pl FORCE
>  	$(call if_changed,perl)
>  
>  clean-files := aesp8-ppc.S ghashp8-ppc.S
> diff --git a/drivers/crypto/vmx/ppc-xlate.pl b/drivers/crypto/vmx/ppc-xlate.pl
> index 36db2ef09e5b..b583898c11ae 100644
> --- a/drivers/crypto/vmx/ppc-xlate.pl
> +++ b/drivers/crypto/vmx/ppc-xlate.pl
> @@ -9,6 +9,8 @@ open STDOUT,">$output" || die "can't open $output: $!";
>  
>  my %GLOBALS;
>  my $dotinlocallabels=($flavour=~/linux/)?1:0;
> +my $elfv2abi=(($flavour =~ /linux-ppc64le/) or ($flavour =~ /linux-ppc64-elfv2/))?1:0;
> +my $dotfunctions=($elfv2abi=~1)?0:1;
>  
>  ################################################################
>  # directives which need special treatment on different platforms
> @@ -40,7 +42,7 @@ my $globl = sub {
>  };
>  my $text = sub {
>      my $ret = ($flavour =~ /aix/) ? ".csect\t.text[PR],7" : ".text";
> -    $ret = ".abiversion	2\n".$ret	if ($flavour =~ /linux.*64le/);
> +    $ret = ".abiversion	2\n".$ret	if ($elfv2abi);
>      $ret;
>  };
>  my $machine = sub {
> @@ -56,8 +58,8 @@ my $size = sub {
>      if ($flavour =~ /linux/)
>      {	shift;
>  	my $name = shift; $name =~ s|^[\.\_]||;
> -	my $ret  = ".size	$name,.-".($flavour=~/64$/?".":"").$name;
> -	$ret .= "\n.size	.$name,.-.$name" if ($flavour=~/64$/);
> +	my $ret  = ".size	$name,.-".($dotfunctions?".":"").$name;
> +	$ret .= "\n.size	.$name,.-.$name" if ($dotfunctions);
>  	$ret;
>      }
>      else
> @@ -142,7 +144,7 @@ my $vmr = sub {
>  
>  # Some ABIs specify vrsave, special-purpose register #256, as reserved
>  # for system use.
> -my $no_vrsave = ($flavour =~ /linux-ppc64le/);
> +my $no_vrsave = ($elfv2abi);
>  my $mtspr = sub {
>      my ($f,$idx,$ra) = @_;
>      if ($idx == 256 && $no_vrsave) {
> -- 
> 2.23.0
>