Re: [PATCH 18/22] x86/traps: Set MSR_PL0_SSP in load_system_tables()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.08.2025 22:23, Andrew Cooper wrote:
> FRED and IDT differ by a Supervisor Token on the base of the shstk.  This
> means that the value they load into MSR_PL0_SSP differs by 8.
> 
> s3_resume() in particular has logic which is otherwise invariant of FRED mode,
> and must not clobber a FRED MSR_PL0_SSP with an IDT one.
> 
> This also simplifies the AP path too.  Updating reinit_bsp_stack() is deferred
> until later.
> 
> No functional change.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

I wonder why this was originally done in assembly in the first place, when
we aim at reducing tghe assembly code we have.

> --- a/xen/arch/x86/boot/x86_64.S
> +++ b/xen/arch/x86/boot/x86_64.S
> @@ -65,17 +65,11 @@ ENTRY(__high_start)
>          or      $(PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 8, %rdx
>  
>          /*
> -         * Write a new supervisor token.  Doesn't matter on boot, but for S3
> -         * resume this clears the busy bit.
> +         * Write a new Supervisor Token.  It doesn't matter the first time a
> +         * CPU boots, but for S3 resume or CPU hot re-add, this clears the
> +         * busy bit.
>           */
>          wrssq   %rdx, (%rdx)
> -
> -        /* Point MSR_PL0_SSP at the token. */
> -        mov     $MSR_PL0_SSP, %ecx
> -        mov     %edx, %eax
> -        shr     $32, %rdx
> -        wrmsr
> -
>          setssbsy

This is ending up a little odd: The comment says the write is to clear the
busy bit, when that's re-set immediately afterwards.

Jan