Re: [PATCH 5/5] x86/alternatives: Discard dynamic check after init

["H. Peter Anvin" <hpa@zytor.com>]

On 01/21/16 10:34, Borislav Petkov wrote:
> From: Brian Gerst <brgerst@gmail.com>
> 
> Move the code to do the dynamic check to the altinstr_aux section so
> that it is discarded after alternatives have run and a static branch has
> been chosen.
> 
> Boris: change it to do TESTB, as hpa suggests.

[Also document that this changes it from C code to assembly, which makes
it *substantially* smaller while avoiding a completely unnecessary call
to an out of line function.]

>   * Static testing of CPU features.  Used the same as boot_cpu_has().
> @@ -141,7 +140,7 @@ extern bool __static_cpu_has(u16 bit);
>  static __always_inline __pure bool _static_cpu_has(u16 bit)
>  {
>  #ifdef CC_HAVE_ASM_GOTO
> -		asm_volatile_goto("1: jmp %l[t_dynamic]\n"
> +		asm_volatile_goto("1: jmp 6f\n"
>  			 "2:\n"
>  			 ".skip -(((5f-4f) - (2b-1b)) > 0) * "
>  			         "((5f-4f) - (2b-1b)),0x90\n"
> @@ -166,13 +165,19 @@ static __always_inline __pure bool _static_cpu_has(u16 bit)
>  			 " .byte 0\n"			/* repl len */
>  			 " .byte 0\n"			/* pad len */
>  			 ".previous\n"
> -			 : : "i" (bit), "i" (X86_FEATURE_ALWAYS)
> -			 : : t_dynamic, t_no);
> +			 ".section .altinstr_aux,\"ax\"\n"
> +			 "6: testb %[bitnum],%[cap_word]\n"
> +			 "   jnz %l[t_yes]\n"
> +			 "   jmp %l[t_no]\n"
> +			 ".previous\n"
> +			 : : "i" (bit), "i" (X86_FEATURE_ALWAYS),
> +			     [bitnum] "i" (1 << (bit & 7)),
> +			     [cap_word] "m" (((const char *)boot_cpu_data.x86_capability)[bit >> 3])

Change to [cap_byte] as we normally refer to this array by 32-bit words.

> +			 : : t_yes, t_no);
> +	t_yes:
>  		return true;
>  	t_no:
>  		return false;
> -	t_dynamic:
> -		return __static_cpu_has(bit);
>  #else
>  		return boot_cpu_has(bit);
>  #endif /* CC_HAVE_ASM_GOTO */
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index a57ec0df9cc3..897c65bd3faa 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1483,12 +1483,6 @@ void warn_pre_alternatives(void)
>  EXPORT_SYMBOL_GPL(warn_pre_alternatives);
>  #endif
>  
> -inline bool __static_cpu_has(u16 bit)
> -{
> -	return boot_cpu_has(bit);
> -}
> -EXPORT_SYMBOL_GPL(__static_cpu_has);
> -
>  static void bsp_resume(void)
>  {
>  	if (this_cpu->c_bsp_resume)
>