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

On 02/09/2018 07:02 PM, Stefano Stabellini wrote:
On Fri, 9 Feb 2018, Julien Grall wrote:

On 02/08/2018 11:49 PM, Stefano Stabellini wrote:
On Thu, 1 Feb 2018, Julien Grall wrote:
On 1 February 2018 at 19:37, Stefano Stabellini <sstabellini@xxxxxxxxxx>
On Tue, 30 Jan 2018, Julien Grall wrote:
Xen does not properly support big.LITTLE platform. All vCPUs of a
will always have the MIDR of the boot CPU (see arch_domain_create).
At best the guest may see unreliable performance (vCPU switching
big and LITTLE), at worst the guest will become unreliable or

This is becoming more apparent with branch predictor hardening in
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.


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


+    /*
+     * 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 (0x%x).\n",
+               smp_processor_id(), current_cpu_data.midr.bits,
+               boot_cpu_data.midr.bits);
+        stop_cpu();
+    }

I understand that this patch is the right thing to do from a correctness
perspective, especially in regards to the SP2 mitigation.

At the same time I would also like to give the option for people that
want to use big.LITTLE with cpupools / cpu pinning to do so if they
really want to, but I am not sure what to suggest.

Could we introduce a command line to proceed anyway? But then the system
would be susceptible to SP2 in the cpus different from the boot cpu.
Could we make the SP2 mitigation work on big.LITTLE or is it too much
trouble? Do you have any other ideas or thoughts about this?

This patch is here to prevent to spread instability/insecurity or give
the feeling we do support big.LITTLE.

Even outside of SP2, there are possibility for instability because CPU
would not be applied correctly in the guest or because Xen is not able to
know that non CPUs may have a different cacheline size...

I want to end this idea that Xen may support big.LITTLE.

The first thing to modify is the vpdir (virtual MIDR), at the moment we
use the boot MIDR. What would you choose now? The MIDR of the CPU where
the hypercall happen?

There is no shortcut for big.LITTLE. The right thing is to implement what
been discussed in the design document written by Dario. But that's a new
feature and would require some work to do it properly.

A command line option might be a good idea, but I would be more of the
to delay that and see who is screaming about it.

My hunch is not many people will scream because today they tend to disable
one set of CPUs in the DT directly.

As discussed, are you going to resend with a command line option such as
biglittle=unsafe or something like that?

I would prefer to avoid term big.LITTLE in the command line option because it
might be possible to have platform with more than two kind of CPUs. How about

I am fine with not using big.LITTLE but smp=unsafe is a bit confusing.
What do you think of: "heterogeneous=unsafe" it is a bit of a mouthful
but it should be clearer.

Heterogeneous does not tell you what you are trying to do. I think it needs to be qualified with the smp (or something similar).\

How about mp_unsafe_heterogeneous=yes/no.


Julien Grall

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