Re: [Xen-devel] PVH CPU hotplug design document

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jan 23, 2017 at 12:05:11PM -0500, Boris Ostrovsky wrote:
> On 01/23/2017 11:50 AM, Roger Pau Monné wrote:
> > On Mon, Jan 23, 2017 at 11:24:19AM -0500, Boris Ostrovsky wrote:
> >> On 01/23/2017 10:58 AM, Jan Beulich wrote:
> >>>>>> On 23.01.17 at 16:49, <boris.ostrovsky@xxxxxxxxxx> wrote:
> >>>> On 01/23/2017 10:43 AM, Boris Ostrovsky wrote:
> >>>>>>> From Linux perspective one option could be to have domU with PV-style
> >>>>>>> vCPU on/offlining based on xenstore and switch to ACPI hotplug if/when
> >>>>>>> it becomes available. This, however, will need an indication from the
> >>>>>>> hypervisor. We could, for example, set ACPI_FADT_HW_REDUCED, as we
> >>>>>>> discussed earlier.
> >>>>>> I think we shouldn't overload that flag. Didn't we settle already on 
> >>>>>> using
> >>>>>> two CPUID flags (of for PV-style onlining/offlining, the other for ACPI
> >>>>>> hot(un)plug)? With that I think I could then be talked into accepting 
> >>>>>> the
> >>>>>> existence of two different models (and kernels could pick which one(s)
> >>>>>> they would like to support).
> >>>>> I forgot about existence of ACPI_FADT_HW_REDUCED until this morning,
> >>>>> which is why I mentioned it now.
> >>>>>
> >>>>> We can go with CPUID flags although I am not sure why we'd need two. I'd
> >>>>> think that OS can be expected to always support PV-style so the flag
> >>>>> would indicate support for ACPI-based hotplug.
> >>>> In fact, it doesn't matter whether OS supports PV-style hotplug. It's
> >>>> that Xen will always set appropriate xenstore entry. It's up to the OS
> >>>> whether to watch it and act upon this.
> >>> That's a good point, perhaps just one CPUID flag will do then indeed.
> >> Roger, are you going to include this in your patchset or do you want me
> >> to do it?
> > I think this should be introduced in your DomU ACPI CPU hotplug series, and 
> > not
> > set for Dom0 until we found a way to perform ACPI vCPU hotplug for Dom0 
> > also.
> >
> 
> Right, although my understanding is that the series is on hold until we
> come to agreement about what to do about dom0. But I guess if we agree
> that we'll have a single ACPI hotplug feature bit then we can go ahead
> with Linux domU patches without waiting for Xen ACPI support.

My understanding is that the way forward now is to introduce this bit, use it
in your series and decide what we do with Dom0 when we get to a point that we
can start to test/implement vCPU hotplug there. There's still work to do until
we can get to the point of simply booting a static no-hotplug PVHv2 Dom0.

Roger.

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