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

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Date:   Wed, 08 Feb 2023 14:13:18 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Brian Gerst <brgerst@...il.com>,
        Usama Arif <usama.arif@...edance.com>
Cc:     dwmw2@...radead.org, 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 Wed, Feb 08 2023 at 00:09, Brian Gerst wrote:
> On Tue, Feb 7, 2023 at 6:10 PM Usama Arif <usama.arif@...edance.com> wrote:
>> 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.

Yes and no. It can be done, but I rather prefer the consistency of the
data and the mechanism. The "overhead" or "win" is not even measurable.

Thanks,

        tglx