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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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.

-boris


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