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From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     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
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.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,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: [PATCHv4 13/14] x86/acpi: Do not attempt to bring up secondary CPUs in kexec case
Date:   Tue,  5 Dec 2023 03:45:09 +0300
Message-ID: <20231205004510.27164-14-kirill.shutemov@linux.intel.com>
In-Reply-To: <20231205004510.27164-1-kirill.shutemov@linux.intel.com>
References: <20231205004510.27164-1-kirill.shutemov@linux.intel.com>
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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>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@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 386adbb03094..5d92d12f1042 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 __ro_afte
 
 static int acpi_wakeup_cpu(u32 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->mailbox_address = 0;
+
 	apic_update_callback(wakeup_secondary_cpu_64, acpi_wakeup_cpu);
 
 	return 0;
-- 
2.41.0