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From:   Brian Gerst <brgerst@gmail.com>
Date:   Wed, 8 Feb 2023 00:09:25 -0500
Message-ID: <CAMzpN2iejCnBeBdC6+92fUL2k8ZdAq_jEgXX+RSoGMhRZ0UBSA@mail.gmail.com>
Subject: Re: [PATCH v7 6/9] x86/smpboot: Support parallel startup of secondary CPUs
To:     Usama Arif <usama.arif@bytedance.com>
Cc:     dwmw2@infradead.org, tglx@linutronix.de, kim.phillips@amd.com,
        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
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On Tue, Feb 7, 2023 at 6:10 PM Usama Arif <usama.arif@bytedance.com> wrote:
>
> From: Thomas Gleixner <tglx@linutronix.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