Re: [PATCH 11/16] x86/spec-ctrl: Adjust DO_OVERWRITE_RSB to be shadow stack compatible

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.05.2020 00:58, Andrew Cooper wrote:
> @@ -114,6 +114,16 @@
>      sub $1, %ecx
>      jnz .L\@_fill_rsb_loop
>      mov %\tmp, %rsp                 /* Restore old %rsp */
> +
> +#ifdef CONFIG_XEN_SHSTK
> +    mov $1, %ecx
> +    rdsspd %ecx
> +    cmp $1, %ecx
> +    je .L\@_shstk_done
> +    mov $64, %ecx                   /* 64 * 4 bytes, given incsspd */
> +    incsspd %ecx                    /* Restore old SSP */
> +.L\@_shstk_done:
> +#endif

The latest here I wonder why you don't use alternatives patching.
I thought that's what you've introduced the synthetic feature
flag for.

Jan