This is a note to let you know that I've just added the patch titled x86/xen: Fix APIC id mismatch warning on Intel to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: x86-xen-fix-apic-id-mismatch-warning-on-intel.patch and it can be found in the queue-4.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From cc272163ea554a97dac180fa8dd6cd54c2810bd1 Mon Sep 17 00:00:00 2001 From: Mohit Gambhir <mohit.gambhir@xxxxxxxxxx> Date: Thu, 26 Jan 2017 13:12:27 -0500 Subject: x86/xen: Fix APIC id mismatch warning on Intel From: Mohit Gambhir <mohit.gambhir@xxxxxxxxxx> commit cc272163ea554a97dac180fa8dd6cd54c2810bd1 upstream. This patch fixes the following warning message seen when booting the kernel as Dom0 with Xen on Intel machines. [0.003000] [Firmware Bug]: CPU1: APIC id mismatch. Firmware: 0 APIC: 1] The code generating the warning in validate_apic_and_package_id() matches cpu_data(cpu).apicid (initialized in init_intel()-> detect_extended_topology() using cpuid) against the apicid returned from xen_apic_read(). Now, xen_apic_read() makes a hypercall to retrieve apicid for the boot cpu but returns 0 otherwise. Hence the warning gets thrown for all but the boot cpu. The idea behind xen_apic_read() returning 0 for apicid is that the guests (even Dom0) should not need to know what physical processor their vcpus are running on. This is because we currently do not have topology information in Xen and also because xen allows more vcpus than physical processors. However, boot cpu's apicid is required for loading xen-acpi-processor driver on AMD machines. Look at following patch for details: commit 558daa289a40 ("xen/apic: Return the APIC ID (and version) for CPU 0.") So to get rid of the warning, this patch modifies xen_cpu_present_to_apicid() to return cpu_data(cpu).apicid instead of calling xen_apic_read(). The warning is not seen on AMD machines because init_amd() populates cpu_data(cpu).apicid by calling hard_smp_processor_id()->xen_apic_read() as opposed to using apicid from cpuid as is done on Intel machines. Signed-off-by: Mohit Gambhir <mohit.gambhir@xxxxxxxxxx> Reviewed-by: Juergen Gross <jgross@xxxxxxxx> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> Cc: Ben Hutchings <ben@xxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/x86/xen/apic.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/arch/x86/xen/apic.c +++ b/arch/x86/xen/apic.c @@ -145,7 +145,7 @@ static void xen_silent_inquire(int apici static int xen_cpu_present_to_apicid(int cpu) { if (cpu_present(cpu)) - return xen_get_apic_id(xen_apic_read(APIC_ID)); + return cpu_data(cpu).apicid; else return BAD_APICID; } Patches currently in stable-queue which might be from mohit.gambhir@xxxxxxxxxx are queue-4.9/x86-xen-fix-apic-id-mismatch-warning-on-intel.patch