Patch "xen/x86: Update topology map for PV VCPUs" has been added to the 4.7-stable tree

This is a note to let you know that I've just added the patch titled

xen/x86: Update topology map for PV VCPUs

to the 4.7-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:
xen-x86-update-topology-map-for-pv-vcpus.patch
and it can be found in the queue-4.7 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Boris Ostrovsky <[email protected]>
Date: Wed, 5 Oct 2016 13:09:33 -0400
Subject: xen/x86: Update topology map for PV VCPUs

From: Boris Ostrovsky <[email protected]>

commit a6a198bc60e6c980a56eca24d33dc7f29139f8ea upstream.

Early during boot topology_update_package_map() computes logical_pkg_ids for all present processors.

Later, when processors are brought up, identify_cpu() updates these values based on phys_pkg_id which is a function of initial_apicid. On PV guests the latter may point to a non-existing node, causing logical_pkg_ids to be set to -1.

Intel's RAPL uses logical_pkg_id (as topology_logical_package_id()) to index its arrays and therefore in this case will point to index 65535 (since logical_pkg_id is a u16). This could lead to either a crash or may actually access random memory location.

As a workaround, we recompute topology during CPU bringup to reset logical_pkg_id to a valid value.

(The reason for initial_apicid being bogus is because it is initial_apicid of the processor from which the guest is launched. This value is CPUID(1).EBX[31:24])

---
 arch/x86/xen/smp.c |    6 ++++++
 1 file changed, 6 insertions(+)

@@ -87,6 +87,12 @@ static void cpu_bringup(void)
 	cpu_data(cpu).x86_max_cores = 1;
 	set_cpu_sibling_map(cpu);
 
+	/*
+	 * identify_cpu() may have set logical_pkg_id to -1 due
+	 * to incorrect phys_proc_id. Let's re-comupte it.
+	 */
+	topology_update_package_map(apic->cpu_present_to_apicid(cpu), cpu);
+
 	xen_setup_cpu_clockevents();
 
 	notify_cpu_starting(cpu);

Patches currently in stable-queue which might be from [email protected] are

queue-4.7/xen-x86-update-topology-map-for-pv-vcpus.patch