Re: [PATCH] x86/xen: Fix APIC id mismatch warning on Intel

From: Boris Ostrovsky
Date: Sun Jan 29 2017 - 22:00:13 EST

On 01/26/2017 01:12 PM, Mohit Gambhir wrote:
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

commit 558daa289a40 ("xen/apic: Return the APIC ID (and version) for CPU

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>

Applied to for-linus-4.11.