Re: [PATCH v2 07/23] x86/boot: Use RSTORSSP to establish SSP

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/09/2025 4:41 pm, Jan Beulich wrote:
> On 01.09.2025 17:33, Andrew Cooper wrote:
>> On 01/09/2025 10:28 am, Jan Beulich wrote:
>>> On 28.08.2025 17:03, Andrew Cooper wrote:
>>>> @@ -908,7 +909,29 @@ static void __init noreturn reinit_bsp_stack(void)
>>>>      if ( cpu_has_xen_shstk )
>>>>      {
>>>>          wrmsrl(MSR_S_CET, xen_msr_s_cet_value());
>>>> -        asm volatile ("setssbsy" ::: "memory");
>>>> +
>>>> +        /*
>>>> +         * IDT and FRED differ by a Supervisor Token on the shadow stack, 
>>>> and
>>>> +         * therefore by the value in MSR_PL0_SSP.
>>> Beside not being overly relevant here afaict, is this last part of the 
>>> sentence
>>> actually correct? Patch 06 doesn't write different values into the MSR.
>> It is correct, but also well hidden.
>>
>> #define MSR_FRED_SSP_SL0                    MSR_PL0_SSP
>>
>> I suppose I should should write MSR_PL0_SSP/MSR_FRED_SSP_SL0 here to
>> highlight the logically different names for the two modes.
> But the code following the comment doesn't access any MSR. That's what
> first tripped me up. It was only then that I wasn't able to spot the two
> different writes. Now that you point out the aliasing it becomes clear
> that until patch 14 it is simply impossible to find that other write.

I suppose the MSR value isn't relevant now that neither paths write the
value.  The first iteration had both writes here.

I guess I can drop that paragraph, and just have the second?

~Andrew