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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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?

-boris


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