Re: [PATCH v4 1/4] x86/speculation: Add __update_spec_ctrl() helper

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 27, 2023 at 10:25:51PM -0400, Waiman Long wrote:
> Add a new __update_spec_ctrl() helper which is a variant of
> update_spec_ctrl() that can be used in a noinstr function.
> 
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  arch/x86/include/asm/nospec-branch.h | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 55388c9f7601..1d363fcea207 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -9,7 +9,7 @@
>  
>  #include <asm/alternative.h>
>  #include <asm/cpufeatures.h>
> -#include <asm/msr-index.h>
> +#include <asm/msr.h>
>  #include <asm/unwind_hints.h>
>  #include <asm/percpu.h>
>  #include <asm/current.h>
> @@ -488,6 +488,15 @@ DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
>  extern void update_spec_ctrl_cond(u64 val);
>  extern u64 spec_ctrl_current(void);
>  
> +/*
> + * This can be used in noinstr function.
> + */
> +static __always_inline void __update_spec_ctrl(u64 val)
> +{
> +	__this_cpu_write(x86_spec_ctrl_current, val);
> +	native_wrmsrl(MSR_IA32_SPEC_CTRL, val);
> +}

Should we also use this to implement update_spec_ctrl() ?