Re: [PATCH v3 01/11] x86/domctl: Add XEN_DOMCTL_set_avail_vcpus

From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: wei.liu2@citrix.com, andrew.cooper3@citrix.com,
	ian.jackson@eu.citrix.com, xen-devel@lists.xen.org,
	Daniel De Graaf <dgdegra@tycho.nsa.gov>,
	roger.pau@citrix.com
Subject: Re: [PATCH v3 01/11] x86/domctl: Add XEN_DOMCTL_set_avail_vcpus
Date: Tue, 22 Nov 2016 09:37:32 -0500	[thread overview]
Message-ID: <e5ef61d9-6156-ffca-77e4-6ea46a9bda09@oracle.com> (raw)
In-Reply-To: <58345D660200007800120CFF@prv-mh.provo.novell.com>

On 11/22/2016 08:59 AM, Jan Beulich wrote:
>>>> On 22.11.16 at 13:34, <boris.ostrovsky@oracle.com> wrote:
>
>>
>> On 11/22/2016 05:39 AM, Jan Beulich wrote:
>>>>>> On 22.11.16 at 11:31, <JBeulich@suse.com> wrote:
>>>>>>> On 21.11.16 at 22:00, <boris.ostrovsky@oracle.com> wrote:
>>>>> This domctl is called when a VCPU is hot-(un)plugged to a guest (via
>>>>> 'xl vcpu-set').
>>>>>
>>>>> The primary reason for adding this call is because for PVH guests
>>>>> the hypervisor needs to send an SCI and set GPE registers. This is
>>>>> unlike HVM guests that have qemu to perform these tasks.
>>>>
>>>> And the tool stack can't do this?
>>>
>>> For the avoidance of further misunderstandings: Of course likely
>>> not completely on its own, but by using a (to be introduced) more
>>> low level hypervisor interface (setting arbitrary GPE bits, with SCI
>>> raised as needed, or the SCI raising being another hypercall).
>>
>> So you are suggesting breaking up XEN_DOMCTL_set_avail_vcpus into
>>
>> XEN_DOMCTL_set_acpi_reg(io_offset, length, val)
>> XEN_DOMCTL_set_avail_vcpus(avail_vcpus_bitmap)
>> XEN_DOMCTL_send_virq(virq)
>>
>> (with perhaps set_avail_vcpus folded into set_acpi_reg) ?
>
> Well, I don't see what set_avail_vcpus would be good for
> considering that during v2 review you've said that you need it
> just for the GPE modification and SCI sending.

Someone needs to provide the hypervisor with the new number of available 
(i.e. hot-plugged/unplugged) VCPUs, thus the name of the domctl. GPE/SCI 
manipulation are part of that update.

(I didn't say it during v2 review and I should have)

>
> And whether to split the other two, or simply send SCI whenever
> GPE bits have been modified depends on the specific requirements
> the ACPI spec puts on us. Or maybe it could always be folded,
> with (if necessary) SCI sending being controlled by a flag.

True. The spec says that all ACPI events generate an SCI.

-boris

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