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Date:   Fri, 27 Oct 2023 16:01:39 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "Kalra, Ashish" <ashish.kalra@amd.com>,
        Sean Christopherson <seanjc@google.com>,
        "Huang, Kai" <kai.huang@intel.com>, Baoquan He <bhe@redhat.com>,
        kexec@lists.infradead.org, linux-coco@lists.linux.dev,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCHv2 11/13] x86/acpi: Do not attempt to bring up secondary
 CPUs in kexec case
Message-ID: <20231027130139.iqvjpu2rizmsxxz7@box.shutemov.name>
References: <20231020151242.1814-1-kirill.shutemov@linux.intel.com>
 <20231020151242.1814-12-kirill.shutemov@linux.intel.com>
 <ebd7c9e8-8bed-423a-a23b-fb93c173cd1b@linux.intel.com>
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On Tue, Oct 24, 2023 at 06:59:58AM -0700, Kuppuswamy Sathyanarayanan wrote:
> 
> 
> 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@linux.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()?

I have not found a cleaner way. Do you have ideas?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov