Xen does not properly support big.LITTLE platform. All vCPUs of a guest
will always have the MIDR of the boot CPU (see arch_domain_create).
At best the guest may see unreliable performance (vCPU switching between
big and LITTLE), at worst the guest will become unreliable or insecure.

This is becoming more apparent with branch predictor hardening in Linux
because they target a specific kind of CPUs and may not work on other

For the time being, park any CPUs with a MDIR different from the boot
CPU. This will be revisited in the future once Xen gains understanding
of big.LITTLE.

[1] https://lists.xenproject.org/archives/html/xen-devel/2016-12/msg00826.html

Signed-off-by: Julien Grall <julien.gr...@arm.com>


We probably want to backport this as part of XSA-254. Using big.LITTLE
on Xen has never been supported but we didn't make it clearly. This is
becoming more apparent with code targeting specific CPUs.

    Changes in v2:
        - Add a command line option to override the default behavior.
 docs/misc/xen-command-line.markdown | 10 ++++++++++
 xen/arch/arm/smpboot.c              | 26 ++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/docs/misc/xen-command-line.markdown 
index 79feba6bcd..cf5997b8db 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1000,6 +1000,16 @@ supported only when compiled with XSM on x86.
 Control Xens use of the APEI Hardware Error Source Table, should one be found.
+### hmp_unsafe (arm)
+> `= <boolean>`
+> Default : `false`
+Say yes at your own risk if you want to enable heterogenous computing
+(such as big.LITTLE). This may result to an unstable and insecure
+platform. When the option is disabled (default), CPUs that are not
+identical to the boot CPU will be parked and not used by Xen.
 ### hpetbroadcast
 > `= <boolean>`
diff --git a/xen/arch/arm/smpboot.c b/xen/arch/arm/smpboot.c
index 1255185a9c..5c05cadb0a 100644
--- a/xen/arch/arm/smpboot.c
+++ b/xen/arch/arm/smpboot.c
@@ -27,6 +27,7 @@
 #include <xen/smp.h>
 #include <xen/softirq.h>
 #include <xen/timer.h>
+#include <xen/warning.h>
 #include <xen/irq.h>
 #include <xen/console.h>
 #include <asm/cpuerrata.h>
@@ -69,6 +70,13 @@ DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_mask);
 /* representing HT and core siblings of each logical CPU */
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_core_mask);
+ * By default non-boot CPUs not identical to the boot CPU will be
+ * parked.
+ */
+static bool __read_mostly opt_hmp_unsafe = false;
+boolean_param("hmp_unsafe", opt_hmp_unsafe);
 static void setup_cpu_sibling_map(int cpu)
     if ( !zalloc_cpumask_var(&per_cpu(cpu_sibling_mask, cpu)) ||
@@ -255,6 +263,9 @@ void __init smp_init_cpus(void)
+    if ( opt_hmp_unsafe )
+                    "It has implications on the security and stability of the 
 int __init
@@ -292,6 +303,21 @@ void start_secondary(unsigned long boot_phys_offset,
+    /*
+     * Currently Xen assumes the platform has only one kind of CPUs.
+     * This assumption does not hold on big.LITTLE platform and may
+     * result to instability and insecure platform. Better to park them
+     * for now.
+     */
+    if ( !opt_hmp_unsafe &&
+         current_cpu_data.midr.bits != boot_cpu_data.midr.bits )
+    {
+        printk(XENLOG_ERR "CPU%u MIDR (0x%x) does not match boot CPU MIDR 
+               smp_processor_id(), current_cpu_data.midr.bits,
+               boot_cpu_data.midr.bits);
+        stop_cpu();
+    }

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