On 01/21/16 10:34, Borislav Petkov wrote:
> From: Brian Gerst <brge...@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)
>
