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

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 15/10/15 10:57, Julien Grall wrote:
> 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.

Hmmm I may not have been clear here. I meant that the interrupt will
stay pending in the distributor but not received by the guest.

-- 
Julien Grall

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