Re: [Xen-devel] [PATCH v2 1/7] x86: Add support for STAC/CLAC instructions

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.04.14 at 16:34, <feng.wu@xxxxxxxxx> wrote:
> @@ -103,4 +105,45 @@ void ret_from_intr(void);
>  
>  #endif
>  
> +/* "Raw" instruction opcodes */
> +#define __ASM_CLAC      .byte 0x0f,0x01,0xca
> +#define __ASM_STAC      .byte 0x0f,0x01,0xcb
> +
> +#ifdef __ASSEMBLY__
> +#define ASM_AC(op)                                       \
> +        pushq %rax;                                      \
> +        leaq boot_cpu_data(%rip), %rax;                  \
> +        btl $X86_FEATURE_SMAP-7*32, CPUINFO86_leaf7_features(%rax);  \
> +        jnc 881f;                                        \
> +        op;                                              \
> +881:    popq %rax

So why are you pushing/popping %rax here? There's no need for the
lea.

And the hard coded 7 here should be replaced too; I don't see a need
for CPUINFO86_leaf7_features either - just calculate everything you
need from X86_FEATURE_SMAP (these are all constants, so other than
the expression getting a little long there's nothing keeping this from
being a single btl).

> +#define ASM_STAC(prefix) ASM_AC(__ASM_STAC, prefix)
> +#define ASM_CLAC(prefix) ASM_AC(__ASM_CLAC, prefix)

What is "prefix" good for here, i.e. why can't you put the % right
in the macro?

Jan


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