Re: [External] Re: [PATCH v12 00/11] Parallel CPU bringup for x86_64

[Usama Arif]

On 27/02/2023 15:29, David Woodhouse wrote:
> On Mon, 2023-02-27 at 06:14 +0000, Usama Arif wrote:
> > diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> > index 17bdd6122dca..5d709aa67df4 100644
> > --- a/arch/x86/kernel/head_64.S
> > +++ b/arch/x86/kernel/head_64.S
> > @@ -273,7 +273,11 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify,
> > SYM_L_GLOBAL)
> >           cmpl    (%rbx,%rcx,4), %edx
> >           jz      .Lsetup_cpu
> >           inc     %ecx
> > +#if (NR_CPUS == 1) || defined(CONFIG_FORCE_NR_CPUS)
> > +       cmpl    $NR_CPUS, %ecx
> > +#else
> >           cmpl    nr_cpu_ids(%rip), %ecx
> > +#endif
> >           jb      .Lfind_cpunr
> >
> >           /*  APIC ID not found in the table. Drop the trampoline lock
> > and bail. */
>
> The whitespace looks dodgy there but maybe that's just your mail client?
>
> Given this code is already in #ifdef CONFIG_SMP, can NR_CPUS be 1?

Ah yes, we have

config NR_CPUS_RANGE_BEGIN
	int
	default NR_CPUS_RANGE_END if MAXSMP
	default    1 if !SMP
	default    2

in arch/x86/Kconfig which doesn't let us select 1 for NR_CPUS if SMP is 
enabled, so this should be enough
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 17bdd6122dca..c79ae67492e1 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -273,7 +273,11 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, 
SYM_L_GLOBAL)
        cmpl    (%rbx,%rcx,4), %edx
        jz      .Lsetup_cpu
        inc     %ecx
+#if defined(CONFIG_FORCE_NR_CPUS)
+       cmpl    $NR_CPUS, %ebx
+#else
        cmpl    nr_cpu_ids(%rip), %ecx
+#endif
        jb      .Lfind_cpunr