Re: [PATCH v2 2/6] x86: reduce CET-SS related #ifdef-ary

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.09.2020 14:30, Jan Beulich wrote:
> Commit b586a81b7a90 ("x86/CET: Fix build following c/s 43b98e7190") had
> to introduce a number of #ifdef-s to make the build work with older tool
> chains. Introduce an assembler macro covering for tool chains not
> knowing of CET-SS, allowing some conditionals where just SETSSBSY is the
> problem to be dropped again.
> 
> No change to generated code.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Now that I've done this I'm no longer sure which direction is better to
> follow: On one hand this introduces dead code (even if just NOPs) into
> CET-SS-disabled builds.

A possible compromise here might be to ...

> --- a/xen/include/asm-x86/asm-defns.h
> +++ b/xen/include/asm-x86/asm-defns.h
> @@ -7,3 +7,9 @@
>      .byte 0x0f, 0x01, 0xcb
>  .endm
>  #endif
> +
> +#ifndef CONFIG_HAS_AS_CET_SS
> +.macro setssbsy
> +    .byte 0xf3, 0x0f, 0x01, 0xe8
> +.endm
> +#endif

... comment out this macro's body. If we went this route, incssp
and wrssp could be dealt with in similar ways, to allow dropping
further #ifdef-s.

Jan