Patch "x86/xen: Fix APIC id mismatch warning on Intel" has been added to the 4.9-stable tree

[<gregkh@linuxfoundation.org>]

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@vger.kernel.org> know about it.


>From cc272163ea554a97dac180fa8dd6cd54c2810bd1 Mon Sep 17 00:00:00 2001
From: Mohit Gambhir <mohit.gambhir@oracle.com>
Date: Thu, 26 Jan 2017 13:12:27 -0500
Subject: x86/xen: Fix APIC id mismatch warning on Intel

From: Mohit Gambhir <mohit.gambhir@oracle.com>

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@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 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@oracle.com are

queue-4.9/x86-xen-fix-apic-id-mismatch-warning-on-intel.patch