Re: [PATCH v3 14/22] x86/boot: choose AP stack based on APIC ID

[Sergii Dmytruk <sergii.dmytruk@xxxxxxxxx>]

On Tue, Jun 30, 2026 at 11:37:59AM +0200, Jan Beulich wrote:
> On 30.06.2026 11:31, Sergii Dmytruk wrote:
> > On Tue, Jun 30, 2026 at 09:17:59AM +0200, Jan Beulich wrote:
> >> On 30.06.2026 00:18, Sergii Dmytruk wrote:
> >>> On Thu, Jan 22, 2026 at 04:52:36PM +0100, Jan Beulich wrote:
> >>>>> --- a/xen/arch/x86/boot/x86_64.S
> >>>>> +++ b/xen/arch/x86/boot/x86_64.S
> >>>>> @@ -15,7 +15,33 @@ ENTRY(__high_start)
> >>>>>          mov     $XEN_MINIMAL_CR4,%rcx
> >>>>>          mov     %rcx,%cr4
> >>>>>
> >>>>> -        mov     stack_start(%rip),%rsp
> >>>>> +        test    %ebx,%ebx
> >>>>> +        cmovz   stack_start(%rip), %rsp
> >>>>> +        jz      .L_stack_set
> >>>>> +
> >>>>> +        /* APs only: get stack base from APIC ID saved in %esp. */
> >>>>> +        mov     $-1, %rax
> >>>>
> >>>> Here and below 32-bit insn would do fine. However, ...
> >>>
> >>> Are all addresses guaraneed to be below 4 GiB?
> >>
> >> You use %rax as the loop induction variable, so there isn't any address
> >> in it? Plus you already use ...
> >>
> >>>>> +        lea     x86_cpu_to_apicid(%rip), %rcx
> >>>>> +1:
> >>>>> +        add     $1, %rax
> >>>>> +        cmp     $NR_CPUS, %eax
> >>
> >> ... as 32-bit insn here anyway.
> >
> > But %rcx contains an address and it is added to %rax for `cmp` (quoted
> > below).
>
> Well, to use SCAS you will need to change how you use registers. (IOW
> first you need to determine whether SCAS actually can be used here,
> and then - if it can - you will need to arrange register use to fit it.)

It can be used, but I don't see any compelling reason to do it.  The
code doesn't get any shorter and has to recover index from a count.

Regards