xen,arm: enable cpu_hotplug

Julien Grall julien.grall at citrix.com
Wed Oct 14 16:23:17 PDT 2015

On 14/10/2015 18:49, Stefano Stabellini wrote:
> Hi all,

Hi Stefano,

> this small patch series enable cpu_hotplug in ARM and ARM64 guests,
> using the PV path to plug and unplug the cpus and psci to enable/disable
> them.

That's a cool things to have on ARM!

I've got few questions related to CPU hotplug on Xen side.

Firstly, when we create the device tree we are using max_vcpus to 
populate the "/cpus" node. AFAIU, Linux will always start all the vCPU 
because they are marked present. That means that it would not be 
possible to honor vcpus="N" where N is < max_vcpus. Did I miss something?

My second point is related to how Xen is handling interrupt with vCPU. 
When PSCI off is called, we will set the _VFP_down flag. This flag is 
used in vgic_vcpu_inject_irq and when it's set the interrupt will be 
ignored and stay active on the HW GIC forever. If the vCPU is coming 
back online, this interrupt will never be received. AFAIU the spec, the 
interrupt is expected to stay pending on the distributor side and will 
be receive when the vCPU will come back or migrate to another vCPU. A 
similar problem can happen when the vCPU is powered on again because we 
clear all the interrupt state related to vCPU (see vgic_clear_pending_irqs).

That brings me a third one related to migration (and not to this series 
specifically). If the interrupt is edge type, it means that same 
interrupt can come while it's still active in the LR register. If the 
guest has to migrate the IRQ it won't happen because the interrupt is 
already active and in the LR, so we will set the pending bit on the 
current vCPU.


Julien Grall

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