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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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