Message-ID: <0f4dc622-6273-8abc-bf9a-d4f737ccb8e9@bytedance.com>
Date:   Mon, 27 Feb 2023 16:32:50 +0000
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.4.2
Subject: Re: [External] Re: [PATCH v12 00/11] Parallel CPU bringup for x86_64
Content-Language: en-US
To:     David Woodhouse <dwmw2@infradead.org>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        tglx@linutronix.de, kim.phillips@amd.com, brgerst@gmail.com
Cc:     piotrgorski@cachyos.org, arjan@linux.intel.com, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com,
        x86@kernel.org, pbonzini@redhat.com, paulmck@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        rcu@vger.kernel.org, mimoja@mimoja.de, hewenliang4@huawei.com,
        thomas.lendacky@amd.com, seanjc@google.com, pmenzel@molgen.mpg.de,
        fam.zheng@bytedance.com, punit.agrawal@bytedance.com,
        simon.evans@bytedance.com, liangma@liangbit.com
References: <20230226110802.103134-1-usama.arif@bytedance.com>
 <5650744.DvuYhMxLoT@natalenko.name>
 <819d8fa2-b73e-e32f-5442-452aa2c0d752@bytedance.com>
 <5e8ad90a-1dc6-95c2-e020-5e95da6f9eda@bytedance.com>
 <90c7efe278fed0616275ed64215ec1fe249f16f5.camel@infradead.org>
From:   Usama Arif <usama.arif@bytedance.com>
In-Reply-To: <90c7efe278fed0616275ed64215ec1fe249f16f5.camel@infradead.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk


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