linux-kernel - Re: [patch v3 08/36] x86/smpboot: Split up native_cpu_up() into separate phases and document them

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Date:   Tue, 9 May 2023 12:19:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        David Woodhouse <dwmw2@...radead.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Brian Gerst <brgerst@...il.com>,
        Arjan van de Veen <arjan@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Paul McKenney <paulmck@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Paul Menzel <pmenzel@...gen.mpg.de>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Piotr Gorski <lucjan.lucjanov@...il.com>,
        Usama Arif <usama.arif@...edance.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        linux-csky@...r.kernel.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, linux-parisc@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        Sabin Rapan <sabrapan@...zon.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [patch v3 08/36] x86/smpboot: Split up native_cpu_up() into
 separate phases and document them


Again, not really this patch, but since I had to look at this code ....

On Mon, May 08, 2023 at 09:43:39PM +0200, Thomas Gleixner wrote:
> @@ -1048,60 +1066,89 @@ static int do_boot_cpu(int apicid, int c

	/*
	 * AP might wait on cpu_callout_mask in cpu_init() with
	 * cpu_initialized_mask set if previous attempt to online
	 * it timed-out. Clear cpu_initialized_mask so that after
	 * INIT/SIPI it could start with a clean state.
	 */
	cpumask_clear_cpu(cpu, cpu_initialized_mask);
	smp_mb();

^^^ that barrier is weird too, cpumask_clear_cpu() is an atomic op and
implies much the same (this is x86 after all). If you want to be super
explicit about it write:

	smp_mb__after_atomic();

(which is a no-op) but then it still very much requires a comment as to
what exactly it orders against what.


	/*
	 * Wake up a CPU in difference cases:
	 * - Use a method from the APIC driver if one defined, with wakeup
	 *   straight to 64-bit mode preferred over wakeup to RM.
	 * Otherwise,
>  	 * - Use an INIT boot APIC message
>  	 */
>  	if (apic->wakeup_secondary_cpu_64)
> +		return apic->wakeup_secondary_cpu_64(apicid, start_ip);
>  	else if (apic->wakeup_secondary_cpu)
> +		return apic->wakeup_secondary_cpu(apicid, start_ip);
>  
> +	return wakeup_secondary_cpu_via_init(apicid, start_ip);
> +}