linux-kernel - Re: [External] Re: [PATCH v7 6/9] x86/smpboot: Support parallel startup of secondary CPUs

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Date:   Wed, 8 Feb 2023 12:54:00 +0000
From:   Usama Arif <usama.arif@...edance.com>
To:     Brian Gerst <brgerst@...il.com>, dwmw2@...radead.org
Cc:     tglx@...utronix.de, kim.phillips@....com, 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 v7 6/9] x86/smpboot: Support parallel
 startup of secondary CPUs



On 08/02/2023 05:09, Brian Gerst wrote:
> On Tue, Feb 7, 2023 at 6:10 PM Usama Arif <usama.arif@...edance.com> wrote:
>>
>> From: Thomas Gleixner <tglx@...utronix.de>
>>
>> Rework the real-mode startup code to allow for APs to be brought up in
>> parallel. This is in two parts:
>>
>> 1. Introduce a bit-spinlock to prevent them from all using the real
>>     mode stack at the same time.
>>
>> 2. Avoid the use of global variables for passing per-CPU information to
>>     the APs.
>>
>> To achieve the latter, export the cpuid_to_apicid[] array so that each
>> AP can find its own per_cpu data (and thus initial_gs, initial_stack and
>> early_gdt_descr) by searching therein based on its APIC ID.
>>
>> Introduce a global variable 'smpboot_control' indicating to the AP how
>> it should find its APIC ID. For a serialized bringup, the APIC ID is
>> explicitly passed in the low bits of smpboot_control, while for parallel
>> mode there are flags directing the AP to find its APIC ID in CPUID leaf
>> 0x0b (for X2APIC mode) or CPUID leaf 0x01 where 8 bits are sufficient.
> 
> For the serialized bringup case, it would be simpler to just put the
> cpu number in the lower bits instead of the APIC id, skipping the
> lookup.
> 
> --
> Brian Gerst

I guess we could do something like below, it would save a few loops 
through find_cpunr in serial case, but probably is as simple as just 
using setup_AP and find_cpunr for all cases? Happy with either but if 
there is a strong preference for below, can change in next revision?

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 656e6018b9d4..30aa543a0114 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -265,7 +265,10 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, 
SYM_L_GLOBAL)
         testl   $STARTUP_APICID_CPUID_01, %edx
         jnz     .Luse_cpuid_01
         andl    $0x0FFFFFFF, %edx
-       jmp     .Lsetup_AP
+       mov     $8, %eax
+       mul     %edx
+       movq    %rax, %rcx
+       jmp     .Linit_cpu_data

  .Luse_cpuid_01:
         mov     $0x01, %eax
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 8ffec5de2e2e..73dd87bf2f29 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1141,7 +1141,7 @@ static int do_boot_cpu(int apicid, int cpu, struct 
task_struct *idle,
                 early_gdt_descr.address = (unsigned 
long)get_cpu_gdt_rw(cpu);
                 initial_stack  = idle->thread.sp;
         } else if (!do_parallel_bringup) {
-               smpboot_control = STARTUP_SECONDARY | apicid;
+               smpboot_control = STARTUP_SECONDARY | cpu;
         }

         /* Enable the espfix hack for this CPU */