Re: [Xen-devel] [PATCH v3 5/4] x86: reduce code size of struct cpu_info member accesses

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Mar 17, 2016 at 10:14:22AM -0600, Jan Beulich wrote:

Something is off with your patch. This is 5/4 :-)

> Instead of addressing these fields via the base of the stack (which
> uniformly requires 4-byte displacements), address them from the end
> (which for everything other than guest_cpu_user_regs requires just
> 1-byte ones). This yields a code size reduction somewhere between 8k
> and 12k in my builds.

Also you made the macro a bit different - the %r is removed.

Particular reason? 

> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Note that just like patch 4 of the series this also isn't directly
> related to the SMEP/SMAP issue, but is again just a result of things
> realized while doing that work, and again depends on the earlier
> patches to apply cleanly.
> 

.. snip..
> --- a/xen/include/asm-x86/asm_defns.h
> +++ b/xen/include/asm-x86/asm_defns.h
> @@ -127,19 +127,19 @@ void ret_from_intr(void);
>          UNLIKELY_DONE(mp, tag);   \
>          __UNLIKELY_END(tag)
>  
> -#define STACK_CPUINFO_FIELD(field) 
> (STACK_SIZE-CPUINFO_sizeof+CPUINFO_##field)
> -#define GET_STACK_BASE(reg)                       \
> -        movq $~(STACK_SIZE-1),reg;                \
> -        andq %rsp,reg
> +#define STACK_CPUINFO_FIELD(field) (1 - CPUINFO_sizeof + CPUINFO_##field)
> +#define GET_STACK_END(reg)                        \
> +        movl $STACK_SIZE-1, %e##reg;              \
> +        orq  %rsp, %r##reg
>  
>  #define GET_CPUINFO_FIELD(field, reg)             \
> -        GET_STACK_BASE(reg);                      \
> -        addq $STACK_CPUINFO_FIELD(field),reg
> +        GET_STACK_END(reg);                       \
> +        addq $STACK_CPUINFO_FIELD(field), %r##reg

Not subq? The GET_STACK_END gets us ..[ edit: missed first time
the change to STACK_CPUINFO_FIELD].


Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

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