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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.01.17 at 16:14, <roger.pau@xxxxxxxxxx> wrote:
> On Fri, Jan 13, 2017 at 08:51:57AM -0700, Jan Beulich wrote:
>> >>> On 12.01.17 at 13:13, <roger.pau@xxxxxxxxxx> wrote:
>> > # Introduction
>> > 
>> > One of the design goals of PVH is to be able to remove as much Xen PV 
>> > specific
>> > code as possible, thus limiting the number of Xen PV interfaces used by 
>> > guests,
>> > and tending to use native interfaces (as used by bare metal) as much as
>> > possible. This is in line with the efforts also done by Xen on ARM and 
>> > helps
>> > reduce the burden of maintaining huge amounts of Xen PV code inside of 
>> > guests
>> > kernels.
>> > 
>> > This however presents some challenges due to the model used by the Xen
>> > Hypervisor, where some devices are handled by Xen while others are left 
>> > for the
>> > hardware domain to manage. The fact that Xen lacks and AML parser also 
>> > makes it
>> > harder, since it cannot get the full hardware description from dynamic ACPI
>> > tables (DSDT, SSDT) without the hardware domain collaboration.
>> 
>> Considering all the difficulties with the proposed model plus the little
>> code the PV vCPU hotplug logic requires in the kernel (assuming a
>> xenbus driver to be there anyway), I'm rather unconvinced that
>> going this route instead of sticking to the PV model is actually
>> desirable. And clearly, for consistency within the kernel, in such a
>> case I'd then also favor sticking to this model for DomU.
> 
> We would at least have to pass the APIC ID in order to perform vCPU hotplug 
> for
> PVH, and the ACPI spec mandates that when using x2APIC structures in the MADT,
> there must be a matching processor object in the DSDT (5.2.12.12).
> 
> Declaring processor objects in the DSDT won't be possible for Xen, but we can
> at least declare them in a SSDT, which seems better than not doing it at all.
> Maybe we can get ACPI to loosen the spec and don't mandate DSDT anymore.

I don't understand this reply of yours: How do any ACPI requirements
come into play when using the PV hotplug mechanism for vCPU-s?

Jan


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