Re: [Xen-devel] xen,arm: enable cpu_hotplug

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Ian,

On 15/10/15 09:39, Ian Campbell wrote:
> On Thu, 2015-10-15 at 00:23 +0100, Julien Grall wrote:
>> 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).
> 
> Is there also an interaction with our implementation of ITARGETSR of
> picking the lowest set bit? e.g. if an IRQ has target 0x6 (targeting CPU 1
> and CPU2) we will choose CPU 1. If CPU 1 is then unplugged, will we end up
> targeting CPU 2 or the now-offline CPU 1? In the latter case the lack of
> interrupts might be considered surprising?

I though about it when I wrote the mail yesterday night.

From the spec section 1.4.3 (ARM IHI 0048B.b):

"The ARM GIC architecture does not guarantee that a 1-N interrupt is
presented to:
â all processors listed in the target processor list
â an enabled interface, where at least one interface is enabled."

AFAIU this paragraph, it means that there is no guarantee to receive an
interrupt if the target mask contain a vCPU offline.

> Or maybe the way CPU hotplug is arranged we never end up with holes in the
> online cpu space, i.e it is not possible to take down CPU1 and leave CPU2
> up?

A kernel is allowed to hotplug any vCPU.

Regards,

-- 
Julien Grall

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