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[Xen-devel] [PATCH 8/9] xen/arm: Park CPUs with a MIDR different from the boot CPU.

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.grall@xxxxxxx>


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.
 xen/arch/arm/smpboot.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/xen/arch/arm/smpboot.c b/xen/arch/arm/smpboot.c
index 1255185a9c..2c2815f9ee 100644
--- a/xen/arch/arm/smpboot.c
+++ b/xen/arch/arm/smpboot.c
@@ -292,6 +292,21 @@ 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 unstability. Better to park them for now.
+     *
+     * TODO: Add big.LITTLE support.
+     */
+    if ( 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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