Re: [PATCH v3 07/22] x86/traps: Alter switch_stack_and_jump() for FRED mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.10.2025 00:53, Andrew Cooper wrote:
> FRED and IDT differ by a Supervisor Token on the base of the shstk.  This
> means that switch_stack_and_jump() needs to discard one extra word when FRED
> is active.
> 
> Fix a typo in the parameter name, which should be shstk_base.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Leave as $%c.  Otherwise it doesn't assemble correctly presented with $$24568
> to parse as an instruction immediate.

I don't follow. Where would the 2nd $ come from if you write ...

> --- a/xen/arch/x86/include/asm/current.h
> +++ b/xen/arch/x86/include/asm/current.h
> @@ -154,7 +154,9 @@ unsigned long get_stack_dump_bottom (unsigned long sp);
>      "rdsspd %[ssp];"                                            \
>      "cmp $1, %[ssp];"                                           \
>      "je .L_shstk_done.%=;" /* CET not active?  Skip. */         \
> -    "mov $%c[skstk_base], %[val];"                              \
> +    ALTERNATIVE("mov $%c[shstk_base], %[val];",                 \
> +                "mov $%c[shstk_base] + 8, %[val];",             \
> +                X86_FEATURE_XEN_FRED)                           \

    ALTERNATIVE("mov %[shstk_base], %[val];",                   \
                "mov %[shstk_base] + 8, %[val];",               \
                X86_FEATURE_XEN_FRED)                           \

Jan