Re: [PATCH 00/10] PVH VCPU hotplug support

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 11/07/2016 06:41 AM, Andrew Cooper wrote:
> On 06/11/16 21:42, Boris Ostrovsky wrote:
>> This series adds support for ACPI-based VCPU hotplug for unprivileged
>> PVH guests.
>>
>> New XEN_DOMCTL_set_avail_vcpus is introduced and is called during
>> guest creation and in response to 'xl vcpu-set' command. This domctl
>> updates GPE0's status and enable registers and sends an SCI to the
>> guest using (newly added) VIRQ_SCI.
> Thankyou for doing this.  Getting APCI hotplug working has been a low
> item on my TODO list for while now.
>
> Some queries and comments however.
>
> This series is currently very PVH centric, to the point of making it
> unusable for plain HVM guests.  While I won't insist on you implementing
> this for HVM (there are some particularly awkward migration problems to
> be considered), I do insist that its implementation isn't tied
> implicitly to being PVH.
>
> The first part of this will be controlling the hypervisor emulation of
> the PM1* blocks with an XEN_X86_EMU_* flag just like all other emulation.

Something like XEN_X86_EMU_ACPI?

That would also eliminate the need for explicitly setting
HVM_PARAM_NR_IOREQ_SERVER_PAGES to zero which I used as indication that
we should have IO handler in the hypervisor. Paul (copied) didn't like that.


>
>>
>> Boris Ostrovsky (10):
>>   x86/domctl: Add XEN_DOMCTL_set_avail_vcpus
> Why is this necessary?  Given that a paravirtual hotplug mechanism
> already exists, why isn't its equivalent mechanism suitable?

PV guests register a xenstore watch and the toolstack updates cpu's 
"available" entry. And ACPI codepath (at least for Linux guest) is not
involved at all.

I don't think we can use anything like that in hypervisor.


>
>>   acpi: Define ACPI IO registers for PVH guests
> Can Xen use pm1b, or does there have to be a pm1a available to the guest?

pm1a is a required block (unlike pm1b). ACPICA, for example, always
first checks pm1a when handling an SCI.

(And how having pm1b only would have helped?)

>
>>   pvh: Set online VCPU map to avail_vcpus
>>   acpi: Power and Sleep ACPI buttons are not emulated
> PVH might not want power/sleep, but you cannot assume that HVM guests
> have a paravirtual mechnism of shutting down.

AFAIK they don't rely on a button-initiated codepath. At least Linux.

I don't know Windows path though. I can add ACPI_HAS_BUTTONS.

>
>>   acpi: Make pmtimer optional in FADT
>>   acpi: PVH guests need _E02 method
> Patch 6 Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
>
>>   pvh/ioreq: Install handlers for ACPI-related PVH IO accesses
> Do not make any assumptions about PVHness based on IOREQ servers.  It
> will not be true for usecases such as vGPU.

Is this comment related to the last patch or is it a general one?  If
it's the latter and we use XEN_X86_EMU_ACPI then I think this will not
be an issue.

-boris



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