linux-kernel - Re: [PATCHv2 11/13] x86/acpi: Do not attempt to bring up secondary CPUs in kexec case

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Date:   Tue, 24 Oct 2023 06:59:58 -0700
From:   Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Kalra, Ashish" <ashish.kalra@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        "Huang, Kai" <kai.huang@...el.com>, Baoquan He <bhe@...hat.com>,
        kexec@...ts.infradead.org, linux-coco@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 11/13] x86/acpi: Do not attempt to bring up secondary
 CPUs in kexec case



On 10/20/2023 8:12 AM, Kirill A. Shutemov wrote:
> ACPI MADT doesn't allow to offline CPU after it got woke up. It limits
> kexec: the second kernel won't be able to use more than one CPU.
> 
> Now acpi_mp_wake_mailbox_paddr already has the mailbox address.
> The acpi_wakeup_cpu() will use it to bring up secondary cpus.
> 
> Zero out mailbox address in the ACPI MADT wakeup structure to indicate
> that the mailbox is not usable.  This prevents the kexec()-ed kernel
> from reading a vaild mailbox, which in turn makes the kexec()-ed kernel
> only be able to use the boot CPU.
> 
> This is Linux-specific protocol and not reflected in ACPI spec.
> 
> Booting the second kernel with signle CPU is enough to cover the most
> common case for kexec -- kdump.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/kernel/acpi/madt_wakeup.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/arch/x86/kernel/acpi/madt_wakeup.c b/arch/x86/kernel/acpi/madt_wakeup.c
> index 4bc1d5106afd..9bbe829737e7 100644
> --- a/arch/x86/kernel/acpi/madt_wakeup.c
> +++ b/arch/x86/kernel/acpi/madt_wakeup.c
> @@ -13,6 +13,11 @@ static struct acpi_madt_multiproc_wakeup_mailbox *acpi_mp_wake_mailbox;
>  
>  static int acpi_wakeup_cpu(int apicid, unsigned long start_ip)
>  {
> +	if (!acpi_mp_wake_mailbox_paddr) {
> +		pr_warn_once("No MADT mailbox: cannot bringup secondary CPUs. Booting with kexec?\n");
> +		return -EOPNOTSUPP;
> +	}
> +
>  	/*
>  	 * Remap mailbox memory only for the first call to acpi_wakeup_cpu().
>  	 *
> @@ -78,6 +83,23 @@ int __init acpi_parse_mp_wake(union acpi_subtable_headers *header,
>  
>  	cpu_hotplug_disable_offlining();
>  
> +	/*
> +	 * ACPI MADT doesn't allow to offline CPU after it got woke up.
> +	 * It limits kexec: the second kernel won't be able to use more than
> +	 * one CPU.
> +	 *
> +	 * Now acpi_mp_wake_mailbox_paddr already has the mailbox address.
> +	 * The acpi_wakeup_cpu() will use it to bring up secondary cpus.
> +	 *
> +	 * Zero out mailbox address in the ACPI MADT wakeup structure to
> +	 * indicate that the mailbox is not usable.  This prevents the
> +	 * kexec()-ed kernel from reading a vaild mailbox, which in turn
> +	 * makes the kexec()-ed kernel only be able to use the boot CPU.
> +	 *
> +	 * This is Linux-specific protocol and not reflected in ACPI spec.
> +	 */
> +	mp_wake->base_address = 0;

Is there any way to skip secondary CPU bring-up for kexec case instead of
returning error in ->wakeup_secondary_cpu_64()?

> +
>  	apic_update_callback(wakeup_secondary_cpu_64, acpi_wakeup_cpu);
>  
>  	return 0;

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer