linux-kernel - Re: [PATCHv6 21/30] x86/acpi, x86/boot: Add multiprocessor wake-up support

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Message-ID: <87zglozr22.ffs@tglx>
Date:   Thu, 17 Mar 2022 13:44:53 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...el.com,
        luto@...nel.org, peterz@...radead.org
Cc:     sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCHv6 21/30] x86/acpi, x86/boot: Add multiprocessor wake-up
 support

On Wed, Mar 16 2022 at 05:08, Kirill A. Shutemov wrote:
> +#ifdef CONFIG_X86_64
> +/* Virtual address of the Multiprocessor Wakeup Structure mailbox */
> +static int acpi_wakeup_cpu(int apicid, unsigned long start_ip)
> +{
> +	static physid_mask_t apic_id_wakemap = PHYSID_MASK_NONE;
> +	u8 timeout;
> +
> +	/* Remap mailbox memory only for the first call to acpi_wakeup_cpu() */
> +	if (physids_empty(apic_id_wakemap)) {

I had to read this condition twice.

Why not checking (!acpi_mp_wake_mailbox)? Too obvious, right?

> +		acpi_mp_wake_mailbox = memremap(acpi_mp_wake_mailbox_paddr,
> +						sizeof(*acpi_mp_wake_mailbox),
> +						MEMREMAP_WB);
> +	}
> +
> +	/*
> +	 * According to the ACPI specification r6.4, section titled
> +	 * "Multiprocessor Wakeup Structure" the mailbox-based wakeup
> +	 * mechanism cannot be used more than once for the same CPU.
> +	 * Skip wakeups if they are attempted more than once.
> +	 */
> +	if (physid_isset(apicid, apic_id_wakemap)) {
> +		pr_err("CPU already awake (APIC ID %x), skipping wakeup\n",
> +		       apicid);
> +		return -EINVAL;
> +	}
> +
> +	spin_lock(&mailbox_lock);

What is this lock actually protecting? Wakeup of secondary CPUs is fully
serialized in the core code already.

Thanks,

        tglx