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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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.


-boris


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