Re: [Xen-devel] [PATCH v2 08/11] pvh/acpi: Handle ACPI accesses for PVH guests

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

>
> When we want to enable ACPI vcpu hotplug for HVM guests, 
>>> What do you mean by "when"? We *are* doing ACPI hotplug for HVM guests,
>>> aren't we?
>> Are we?  If so, how?
>>
>> I don't see any toolstack or qemu code able to cope with APCI CPU
>> hotplug.  I can definitely see ACPI PCI hotplug in qemu, but that does
>> make sense.
> piix4_acpi_system_hot_add_init():
>    acpi_cpu_hotplug_init(parent, OBJECT(s), &s->gpe_cpu,
>                             PIIX4_CPU_HOTPLUG_IO_BASE);
>
>
>>> Or are you thinking about moving this functionality to the hypervisor?
>> As an aside, we need to move some part of PCI hotplug into the
>> hypervisor longterm.  At the moment, any new entity coming along and
>> attaching to an ioreq server still needs to negotiate with Qemu to make
>> the device appear.  This is awkward but doable if all device models are
>> in dom0, but is far harder if the device models are in different domains.
>>
>> As for CPU hotplug, (if I have indeed overlooked something), Qemu has no
>> business in this matter. 
> Yes. And if we are going to do it for PVH we might as well do it for HVM
> --- I think most of the code will be the same, save for how SCI is sent.


So I discovered that we actually cannot unplug an HVM VCPU with qemu,
there is no support for that via QMP (which is what we use).

'xl vcpu-set <domid> N' is nop when we unplug.


-boris


>
> -boris
>
>>  The device model exists to be an
>> implementation of an LPC bridge, and is not responsible for any CPU
>> related functionality; Xen does all vcpu handling.
>>
>>
>> The Xen project and community have had a very rich history of hacking
>> things up in the past, an frankly, it shows.  I want to ensure that
>> development progresses in an architecturally clean and appropriate
>> direction, especially if this enables us to remove some of the duck tape
>> holding pre-existing features together.
>>
>> ~Andrew
>



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