linux-kernel - Re: [External] Re: [PATCH v12 00/11] Parallel CPU bringup for x86

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Date:   Mon, 27 Feb 2023 16:32:50 +0000
From:   Usama Arif <usama.arif@...edance.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        tglx@...utronix.de, kim.phillips@....com, brgerst@...il.com
Cc:     piotrgorski@...hyos.org, arjan@...ux.intel.com, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        x86@...nel.org, pbonzini@...hat.com, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        thomas.lendacky@....com, seanjc@...gle.com, pmenzel@...gen.mpg.de,
        fam.zheng@...edance.com, punit.agrawal@...edance.com,
        simon.evans@...edance.com, liangma@...ngbit.com
Subject: Re: [External] Re: [PATCH v12 00/11] Parallel CPU bringup for x86_64



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