[Xen-devel] [PATCH] xen/x86: Update topology map for PV VCPUs

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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])

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---

Copying Andrew for the CPUID part.

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

diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index 311acad..9fa27ce 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -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);
-- 
1.8.3.1


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