Re: [PATCH 3/3] x86: Use alternative mechanism to define CLAC/STAC

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 26/05/2014 08:27, Feng Wu wrote:
> This patch use alternative mechanism to define CLAC/STAC.
>
> Signed-off-by: Feng Wu <feng.wu@intel.com>
> ---
>  xen/include/asm-x86/asm_defns.h | 36 +++++++++++++++++++++++-------------
>  1 file changed, 23 insertions(+), 13 deletions(-)
>
> diff --git a/xen/include/asm-x86/asm_defns.h b/xen/include/asm-x86/asm_defns.h
> index 87a462f..92024cc 100644
> --- a/xen/include/asm-x86/asm_defns.h
> +++ b/xen/include/asm-x86/asm_defns.h
> @@ -10,6 +10,8 @@
>  #include <asm/percpu.h>
>  #include <xen/stringify.h>
>  #include <asm/cpufeature.h>
> +#include <asm/nops.h>
> +#include <asm/alternative.h>

alternatives should include nops, to avoid needing a double include
everywhere alternative() is used.

>  
>  #ifndef __ASSEMBLY__
>  void ret_from_intr(void);
> @@ -166,26 +168,34 @@ void ret_from_intr(void);
>  #define __ASM_STAC      .byte 0x0f,0x01,0xcb
>  
>  #ifdef __ASSEMBLY__
> -#define ASM_AC(op)                                       \
> -        btl $X86_FEATURE_SMAP & 31,                      \
> -        CPUINFO_FEATURE_OFFSET(X86_FEATURE_SMAP)+boot_cpu_data(%rip); \
> -        jnc 881f;                                        \
> -        __ASM_##op;                                      \
> -881:
> -
> -#define ASM_STAC ASM_AC(STAC)
> -#define ASM_CLAC ASM_AC(CLAC)
> +#define ASM_CLAC                                                       \
> +        661: ASM_NOP3;                                                 \
> +        .pushsection .altinstr_replacement, "ax";                      \

The altinstr_replacement section doesn't need to be executable.  The
data inside it is copied out into the .text section.

> +        662: __ASM_CLAC;                                               \
> +        .popsection;                                                   \
> +        .pushsection .altinstructions, "a";                            \
> +        altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3;       \
> +        .popsection

these 661s and 662s alias magic numbers in the alternatives core code,
which could lead to subtle issues.  Surely there is a better way than
having to choose magic numbers which don't collide with anything else in
the source.

~Andrew

> +
> +#define ASM_STAC                                                       \
> +        661: ASM_NOP3;                                                 \
> +        .pushsection .altinstr_replacement, "ax";                      \
> +        662: __ASM_STAC;                                               \
> +        .popsection;                                                   \
> +        .pushsection .altinstructions, "a";                            \
> +        altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3;       \
> +        .popsection
>  #else
>  static inline void clac(void)
>  {
> -    if ( boot_cpu_has(X86_FEATURE_SMAP) )
> -        asm volatile (__stringify(__ASM_CLAC) : : : "memory");
> +    /* Note: a barrier is implicit in alternative() */
> +    alternative(ASM_NOP3, __stringify(__ASM_CLAC), X86_FEATURE_SMAP);
>  }
>  
>  static inline void stac(void)
>  {
> -    if ( boot_cpu_has(X86_FEATURE_SMAP) )
> -        asm volatile (__stringify(__ASM_STAC) : : : "memory");
> +    /* Note: a barrier is implicit in alternative() */
> +    alternative(ASM_NOP3, __stringify(__ASM_STAC), X86_FEATURE_SMAP);
>  }
>  #endif
>