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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 07/10/2025 4:58 pm, Jan Beulich wrote:
> 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)                           \

I find this feedback completely uncharacteristic.  You always goes out
of your way to hide % inside macros to prohibit non-register operands.

This is exactly the same, except to force an immediate operand, so the
length of the two instructions is the same.

~Andrew