Re: [PATCH 1/6] x86/entry: Introduce local_db_{save,restore}()

[Andrew Cooper]

On 28/05/2020 21:19, Peter Zijlstra wrote:
> --- a/arch/x86/include/asm/debugreg.h
> +++ b/arch/x86/include/asm/debugreg.h
> @@ -113,6 +113,31 @@ static inline void debug_stack_usage_inc
>  static inline void debug_stack_usage_dec(void) { }
>  #endif /* X86_64 */
>  
> +static __always_inline void local_db_save(unsigned long *dr7)
> +{
> +	get_debugreg(*dr7, 7);
> +	if (*dr7)
> +		set_debugreg(0, 7);

%dr7 has an architecturally stuck bit in it.

You want *dr7 != 0x400 to avoid writing 0 unconditionally.

Also, API wise, wouldn't it be nicer to write "dr7 = local_db_save()"
rather than having a void function returning a single long via pointer?

~Andrew