[PATCH v3] MIPS: Workaround clang inline compat branch issue

[PATCH v3] MIPS: Workaround clang inline compat branch issue

[Jiaxun Yang]

Clang is unable to handle the situation that a chunk of inline
assembly ends with a compat branch instruction and then compiler
generates another control transfer instruction immediately after
this compat branch. The later instruction will end up in forbidden
slot and cause exception.

Workaround by add a option to control the use of compact branch.
Currently it's selected by CC_IS_CLANG and hopefully we can change
it to a version check in future if clang manages to fix it.

Fix boot on boston board.

Link: https://github.com/llvm/llvm-project/issues/61045
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Acked-by: Nathan Chancellor <nathan@kernel.org>
---
v2: Add Link tag to LLVM bug
v3: Docuement issue link in Kconfig as well
---
 arch/mips/Kconfig           | 4 ++++
 arch/mips/include/asm/asm.h | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 37072e15b263..c8b878aa690f 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -3206,6 +3206,10 @@ config CC_HAS_MNO_BRANCH_LIKELY
 	def_bool y
 	depends on $(cc-option,-mno-branch-likely)
 
+# https://github.com/llvm/llvm-project/issues/61045
+config CC_HAS_BROKEN_INLINE_COMPAT_BRANCH
+	def_bool y if CC_IS_CLANG
+
 menu "Power management options"
 
 config ARCH_HIBERNATION_POSSIBLE
diff --git a/arch/mips/include/asm/asm.h b/arch/mips/include/asm/asm.h
index 336ac9b65235..2e99450f4228 100644
--- a/arch/mips/include/asm/asm.h
+++ b/arch/mips/include/asm/asm.h
@@ -336,7 +336,7 @@ symbol		=	value
  */
 #ifdef CONFIG_WAR_R10000_LLSC
 # define SC_BEQZ	beqzl
-#elif MIPS_ISA_REV >= 6
+#elif !defined(CONFIG_CC_HAS_BROKEN_INLINE_COMPAT_BRANCH) && MIPS_ISA_REV >= 6
 # define SC_BEQZ	beqzc
 #else
 # define SC_BEQZ	beqz
-- 
2.37.1 (Apple Git-137.1)

Re: [PATCH v3] MIPS: Workaround clang inline compat branch issue

[Nick Desaulniers]

On Tue, Feb 28, 2023 at 11:35 AM Jiaxun Yang <jiaxun.yang@flygoat.com> wrote:
>
> Clang is unable to handle the situation that a chunk of inline
> assembly ends with a compat branch instruction and then compiler
> generates another control transfer instruction immediately after
> this compat branch. The later instruction will end up in forbidden
> slot and cause exception.
>
> Workaround by add a option to control the use of compact branch.
> Currently it's selected by CC_IS_CLANG and hopefully we can change
> it to a version check in future if clang manages to fix it.
>
> Fix boot on boston board.
>
> Link: https://github.com/llvm/llvm-project/issues/61045
> Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
> Acked-by: Nathan Chancellor <nathan@kernel.org>

Thanks for the patch!

Acked-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
> v2: Add Link tag to LLVM bug
> v3: Docuement issue link in Kconfig as well
> ---
>  arch/mips/Kconfig           | 4 ++++
>  arch/mips/include/asm/asm.h | 2 +-
>  2 files changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index 37072e15b263..c8b878aa690f 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -3206,6 +3206,10 @@ config CC_HAS_MNO_BRANCH_LIKELY
>         def_bool y
>         depends on $(cc-option,-mno-branch-likely)
>
> +# https://github.com/llvm/llvm-project/issues/61045
> +config CC_HAS_BROKEN_INLINE_COMPAT_BRANCH
> +       def_bool y if CC_IS_CLANG
> +
>  menu "Power management options"
>
>  config ARCH_HIBERNATION_POSSIBLE
> diff --git a/arch/mips/include/asm/asm.h b/arch/mips/include/asm/asm.h
> index 336ac9b65235..2e99450f4228 100644
> --- a/arch/mips/include/asm/asm.h
> +++ b/arch/mips/include/asm/asm.h
> @@ -336,7 +336,7 @@ symbol              =       value
>   */
>  #ifdef CONFIG_WAR_R10000_LLSC
>  # define SC_BEQZ       beqzl
> -#elif MIPS_ISA_REV >= 6
> +#elif !defined(CONFIG_CC_HAS_BROKEN_INLINE_COMPAT_BRANCH) && MIPS_ISA_REV >= 6
>  # define SC_BEQZ       beqzc
>  #else
>  # define SC_BEQZ       beqz
> --
> 2.37.1 (Apple Git-137.1)
>
>


-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v3] MIPS: Workaround clang inline compat branch issue

[Thomas Bogendoerfer]

On Tue, Feb 28, 2023 at 07:34:59PM +0000, Jiaxun Yang wrote:
> Clang is unable to handle the situation that a chunk of inline
> assembly ends with a compat branch instruction and then compiler
> generates another control transfer instruction immediately after
> this compat branch. The later instruction will end up in forbidden
> slot and cause exception.
> 
> Workaround by add a option to control the use of compact branch.
> Currently it's selected by CC_IS_CLANG and hopefully we can change
> it to a version check in future if clang manages to fix it.
> 
> Fix boot on boston board.
> 
> Link: https://github.com/llvm/llvm-project/issues/61045
> Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
> Acked-by: Nathan Chancellor <nathan@kernel.org>
> ---
> v2: Add Link tag to LLVM bug
> v3: Docuement issue link in Kconfig as well
> ---
>  arch/mips/Kconfig           | 4 ++++
>  arch/mips/include/asm/asm.h | 2 +-
>  2 files changed, 5 insertions(+), 1 deletion(-)

applied to mips-next.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]

Re: [PATCH v3] MIPS: Workaround clang inline compat branch issue

[Philippe Mathieu-Daudé]

On 28/2/23 20:34, Jiaxun Yang wrote:
> Clang is unable to handle the situation that a chunk of inline
> assembly ends with a compat branch instruction and then compiler
> generates another control transfer instruction immediately after
> this compat branch. The later instruction will end up in forbidden
> slot and cause exception.
> 
> Workaround by add a option to control the use of compact branch.
> Currently it's selected by CC_IS_CLANG and hopefully we can change
> it to a version check in future if clang manages to fix it.
> 
> Fix boot on boston board.
> 
> Link: https://github.com/llvm/llvm-project/issues/61045
> Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
> Acked-by: Nathan Chancellor <nathan@kernel.org>
> ---
> v2: Add Link tag to LLVM bug
> v3: Docuement issue link in Kconfig as well
> ---
>   arch/mips/Kconfig           | 4 ++++
>   arch/mips/include/asm/asm.h | 2 +-
>   2 files changed, 5 insertions(+), 1 deletion(-)

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>