Re: [PATCH v2 08/23] x86/traps: Alter switch_stack_and_jump() for FRED mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.08.2025 17:03, 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.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

However, I'd much prefer if ...

> --- 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[skstk_base], %[val];",                 \
> +                "mov $%c[skstk_base] + 8, %[val];",             \

... the unnecessarily complicated $%c here could be replaced by plain %.

Jan

> +                X86_FEATURE_XEN_FRED)                           \
>      "and $%c[stack_mask], %[ssp];"                              \
>      "sub %[ssp], %[val];"                                       \
>      "shr $3, %[val];"                                           \