linux-kernel - Re: [PATCH 3/3] riscv: Select HAVE_DYNAMIC_FTRACE when -fpatchable-function-entry is available

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Date:   Thu, 25 Mar 2021 16:38:35 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 3/3] riscv: Select HAVE_DYNAMIC_FTRACE when
 -fpatchable-function-entry is available

On 2021-03-25, Nathan Chancellor wrote:
>clang prior to 13.0.0 does not support -fpatchable-function-entry for
>RISC-V.
>
>clang: error: unsupported option '-fpatchable-function-entry=8' for target 'riscv64-unknown-linux-gnu'
>
>To avoid this error, only select HAVE_DYNAMIC_FTRACE when this option is
>not available.

If clang -fpatchable-function-entry=8 does not error "unsupported
option" for one target, it means the backend feature is supported on
this target.

Reviewed-by: Fangrui Song <maskray@...gle.com>

>Fixes: afc76b8b8011 ("riscv: Using PATCHABLE_FUNCTION_ENTRY instead of MCOUNT")
>Link: https://github.com/ClangBuiltLinux/linux/issues/1268
>Reported-by: kernel test robot <lkp@...el.com>
>Signed-off-by: Nathan Chancellor <nathan@...nel.org>
>---
> arch/riscv/Kconfig | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>index 87d7b52f278f..ba1d07640b66 100644
>--- a/arch/riscv/Kconfig
>+++ b/arch/riscv/Kconfig
>@@ -227,7 +227,7 @@ config ARCH_RV64I
> 	bool "RV64I"
> 	select 64BIT
> 	select ARCH_SUPPORTS_INT128 if CC_HAS_INT128 && GCC_VERSION >= 50000
>-	select HAVE_DYNAMIC_FTRACE if MMU
>+	select HAVE_DYNAMIC_FTRACE if MMU && $(cc-option,-fpatchable-function-entry=8)
> 	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
> 	select HAVE_FTRACE_MCOUNT_RECORD
> 	select HAVE_FUNCTION_GRAPH_TRACER
>-- 
>2.31.0
>
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