Re: [PATCH v2 2/4] arm64: vdso32: Detect binutils support for dmb ishld

[Nick Desaulniers]

On Thu, Sep 26, 2019 at 6:38 AM Vincenzo Frascino
<vincenzo.frascino@xxxxxxx> wrote:
>
> As reported by Will Deacon, older versions of binutils that do not
> support certain types of memory barriers can cause build failure of the
> vdso32 library.

Do we know which specific version of binutils has this problem?
Documentation/process/changes.rst lists 2.21+ as the supported
versions.  If it's older than that, it's unsupported.

I triple checked android's 2.27 seems ok.

$ cat bar.s
.text
.globl foo
  dmb ish
  dmb ishld
  dmb ishst
$ aarch64-linux-gnu-as bar.s
$ echo $?
0
$ llvm-objdump -d a.out

a.out: file format ELF64-aarch64-little


Disassembly of section .text:

0000000000000000 $x:
       0: bf 3b 03 d5                  dmb ish
       4: bf 39 03 d5                  dmb ishld
       8: bf 3a 03 d5                  dmb ishst

>
> Add a compilation time mechanism that detects if binutils supports those
> instructions and configure the kernel accordingly.
>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> ---
>  arch/arm64/Kbuild                            | 6 ++++++
>  arch/arm64/include/asm/vdso/compat_barrier.h | 2 +-
>  arch/arm64/kernel/vdso32/Makefile            | 9 +++++++++
>  3 files changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/Kbuild b/arch/arm64/Kbuild
> index d6465823b281..75cf8c796d0e 100644
> --- a/arch/arm64/Kbuild
> +++ b/arch/arm64/Kbuild
> @@ -4,3 +4,9 @@ obj-$(CONFIG_NET)       += net/
>  obj-$(CONFIG_KVM)      += kvm/
>  obj-$(CONFIG_XEN)      += xen/
>  obj-$(CONFIG_CRYPTO)   += crypto/
> +
> +# as-instr-compat
> +# Usage: cflags-y += $(call as-instr-compat,instr,option1,option2)
> +
> +as-instr-compat = $(call try-run,\
> +       printf "%b\n" "$(1)" | $(COMPATCC) $(KBUILD_AFLAGS) -c -x assembler -o "$$TMP" -,$(2),$(3))
> diff --git a/arch/arm64/include/asm/vdso/compat_barrier.h b/arch/arm64/include/asm/vdso/compat_barrier.h
> index fb60a88b5ed4..3fd8fd6d8fc2 100644
> --- a/arch/arm64/include/asm/vdso/compat_barrier.h
> +++ b/arch/arm64/include/asm/vdso/compat_barrier.h
> @@ -20,7 +20,7 @@
>
>  #define dmb(option) __asm__ __volatile__ ("dmb " #option : : : "memory")
>
> -#if __LINUX_ARM_ARCH__ >= 8
> +#if __LINUX_ARM_ARCH__ >= 8 && defined(CONFIG_AS_DMB_ISHLD)
>  #define aarch32_smp_mb()       dmb(ish)
>  #define aarch32_smp_rmb()      dmb(ishld)
>  #define aarch32_smp_wmb()      dmb(ishst)
> diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> index 1fba0776ed40..22f0d31ea528 100644
> --- a/arch/arm64/kernel/vdso32/Makefile
> +++ b/arch/arm64/kernel/vdso32/Makefile
> @@ -17,6 +17,8 @@ cc32-disable-warning = $(call try-run,\
>         $(COMPATCC) -W$(strip $(1)) -c -x c /dev/null -o "$$TMP",-Wno-$(strip $(1)))
>  cc32-ldoption = $(call try-run,\
>          $(COMPATCC) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
> +cc32-as-instr = $(call try-run,\
> +       printf "%b\n" "$(1)" | $(COMPATCC) $(VDSO_AFLAGS) -c -x assembler -o "$$TMP" -,$(2),$(3))
>
>  # We cannot use the global flags to compile the vDSO files, the main reason
>  # being that the 32-bit compiler may be older than the main (64-bit) compiler
> @@ -55,6 +57,7 @@ endif
>  VDSO_CAFLAGS += -fPIC -fno-builtin -fno-stack-protector
>  VDSO_CAFLAGS += -DDISABLE_BRANCH_PROFILING
>
> +
>  # Try to compile for ARMv8. If the compiler is too old and doesn't support it,
>  # fall back to v7. There is no easy way to check for what architecture the code
>  # is being compiled, so define a macro specifying that (see arch/arm/Makefile).
> @@ -91,6 +94,12 @@ VDSO_CFLAGS += -Wno-int-to-pointer-cast
>  VDSO_AFLAGS := $(VDSO_CAFLAGS)
>  VDSO_AFLAGS += -D__ASSEMBLY__
>
> +# Check for binutils support for dmb ishld
> +dmbinstr := $(call cc32-as-instr,dmb ishld,-DCONFIG_AS_DMB_ISHLD=1)
> +
> +VDSO_CFLAGS += $(dmbinstr)
> +VDSO_AFLAGS += $(dmbinstr)
> +
>  VDSO_LDFLAGS := $(VDSO_CPPFLAGS)
>  # From arm vDSO Makefile
>  VDSO_LDFLAGS += -Wl,-Bsymbolic -Wl,--no-undefined -Wl,-soname=linux-vdso.so.1
> --
> 2.23.0
>


-- 
Thanks,
~Nick Desaulniers