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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 15 Oct 2015, 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.

Ah good! Our do_psci_cpu_off implementation is OK after all :-)


> > 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.

Yes, but xl only allowed to hot(un)plug cpus in order, look up vcpu-set
on man xl.

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