linux-kernel - Re: [PATCH v6 06/11] x86/smpboot: Support parallel startup of secondary CPUs

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Date:   Tue, 07 Feb 2023 01:07:45 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Usama Arif <usama.arif@...edance.com>, dwmw2@...radead.org,
        arjan@...ux.intel.com
Cc:     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, Usama Arif <usama.arif@...edance.com>
Subject: Re: [PATCH v6 06/11] x86/smpboot: Support parallel startup of
 secondary CPUs

On Thu, Feb 02 2023 at 21:56, Usama Arif wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
>
> To allow for parallel AP bringup, we need to avoid the use of global
> variables for passing information to the APs, as well as preventing them
> from all trying to use the same real-mode stack simultaneously.
>
> So, introduce a 'lock' field in struct trampoline_header to use as a
> simple bit-spinlock for the real-mode stack. That lock also protects
> the global variables initial_gs, initial_stack and early_gdt_descr,
> which can now be calculated...
>
> So how do we calculate those addresses? Well, they they can all be found
> from the per_cpu data for this CPU. Simples! Except... how does it know
> what its CPU# is? OK, we export the cpuid_to_apicid[] array and it can
> search it to find its APIC ID in there.
>
> But now you whine at me that it doesn't even know its APIC ID? Well, if
> it's a relatively modern CPU then the APIC ID is in CPUID leaf 0x0B so
> we can use that. Otherwise... erm... OK, otherwise it can't have parallel
> CPU bringup for now. We'll still use a global variable for those CPUs and
> bring them up one at a time.
>
> So add a global 'smpboot_control' field which either contains the APIC
> ID, or a flag indicating that it can be found in CPUID.
>
> This adds the 'do_parallel_bringup' flag in preparation but doesn't
> actually enable parallel bringup yet.
>
> [ dwmw2: Minor tweaks, write a commit message ]
> [ seanc: Fix stray override of initial_gs in common_cpu_up() ]
> Not-signed-off-by: Thomas Gleixner <tglx@...utronix.de>

I'm happy to add my SOB if someone actually writes a coherent changelog
which complies with the documented rules :)