Re: [PATCH 2/3] x86/clang: add retpoline support

[Jan Beulich <jbeulich@xxxxxxxx>]

On 16.02.2022 10:02, Roger Pau Monne wrote:
> Detect whether the compiler supports clang retpoline option and enable
> by default if available, just like it's done for gcc.
> 
> Note clang already disables jump tables when retpoline is enabled, so
> there's no need to also pass the fno-jump-tables parameter.

That's one of the secondary gcc side arrangements. What about the other
(-mindirect-branch-register)?

> Reported-by: Jan Beulich <JBeulich@xxxxxxxx>

That's Andrew, not me?

Jan

> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
>  xen/arch/x86/Kconfig | 5 ++++-
>  xen/arch/x86/arch.mk | 3 +++
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
> index 2cd713724f..fe00b4598b 100644
> --- a/xen/arch/x86/Kconfig
> +++ b/xen/arch/x86/Kconfig
> @@ -35,8 +35,11 @@ config ARCH_DEFCONFIG
>  config GCC_INDIRECT_THUNK
>       def_bool $(cc-option,-mindirect-branch-register)
>  
> +config CLANG_INDIRECT_THUNK
> +     def_bool $(cc-option,-mretpoline-external-thunk)
> +
>  config INDIRECT_THUNK
> -     depends on GCC_INDIRECT_THUNK
> +     depends on GCC_INDIRECT_THUNK || CLANG_INDIRECT_THUNK
>       def_bool y
>  
>  config HAS_AS_CET_SS
> diff --git a/xen/arch/x86/arch.mk b/xen/arch/x86/arch.mk
> index 2da4bdb1ed..f2aa2a515f 100644
> --- a/xen/arch/x86/arch.mk
> +++ b/xen/arch/x86/arch.mk
> @@ -47,6 +47,9 @@ CFLAGS-$(CONFIG_GCC_INDIRECT_THUNK) += 
> -mindirect-branch=thunk-extern
>  CFLAGS-$(CONFIG_GCC_INDIRECT_THUNK) += -mindirect-branch-register
>  CFLAGS-$(CONFIG_GCC_INDIRECT_THUNK) += -fno-jump-tables
>  
> +# Enable clang retpoline support if available.
> +CFLAGS-$(CONFIG_CLANG_INDIRECT_THUNK) += -mretpoline-external-thunk
> +
>  # If supported by the compiler, reduce stack alignment to 8 bytes. But allow
>  # this to be overridden elsewhere.
>  $(call cc-option-add,CFLAGS_stack_boundary,CC,-mpreferred-stack-boundary=3)