linux-kernel - Re: [PATCH v3 6/9] x86/smpboot: Support parallel startup of secondary CPUs

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Date:   Tue, 1 Feb 2022 11:56:08 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "mimoja@...oja.de" <mimoja@...oja.de>,
        "hewenliang4@...wei.com" <hewenliang4@...wei.com>,
        "hushiyuan@...wei.com" <hushiyuan@...wei.com>,
        "luolongjun@...wei.com" <luolongjun@...wei.com>,
        "hejingxian@...wei.com" <hejingxian@...wei.com>,
        Joerg Roedel <joro@...tes.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v3 6/9] x86/smpboot: Support parallel startup of
 secondary CPUs

On Tue, Feb 01, 2022 at 10:25:01AM +0000, David Woodhouse wrote:
> Thanks. It looks like that is only invoked after boot, with a write to
> /sys/devices/system/cpu/microcode/reload.
>
> My series is only parallelising the initial bringup at boot time, so it
> shouldn't make any difference.

No, I don't mean __reload_late() - I pointed you at that function to
show the dance we must do when updating microcode late.

The load_ucode_{ap,bsp}() routines are what is called when loading ucode
early.

So the question is, does the parallelizing change the order in which APs
are brought up and can it happen that a SMT sibling of a two-SMT core
executes *something* while the other SMT sibling is updating microcode.

If so, that would be bad.

> However... it does look like there's nothing preventing a sibling being
> brought online *while* the dance you mention above is occurring.

Bottom line is: of the two SMT siblings, one needs to be updating
microcode while the other is idle. I.e., what __reload_late() does.

> Shouldn't __reload_late() take the device_hotplug_lock to prevent that?

See reload_store().

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette