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

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Date:   Wed, 8 Feb 2023 00:09:25 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     Usama Arif <usama.arif@...edance.com>
Cc:     dwmw2@...radead.org, 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: [PATCH v7 6/9] x86/smpboot: Support parallel startup of secondary CPUs

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