Re: [Xen-devel] "xl vcpu-set" not persistent across reboot?

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/06/16 18:20, Wei Liu wrote:
> Use Stefano's new email address
>
> On Mon, Jun 06, 2016 at 06:18:06PM +0100, Wei Liu wrote:
>> On Fri, Jun 03, 2016 at 05:35:20PM +0100, Wei Liu wrote:
>>> On Fri, Jun 03, 2016 at 08:42:11AM -0600, Jan Beulich wrote:
>>>>>>> On 03.06.16 at 15:41, <wei.liu2@xxxxxxxxxx> wrote:
>>>>> On Fri, Jun 03, 2016 at 02:29:12AM -0600, Jan Beulich wrote:
>>>>>> Ian, Wei,
>>>>>>
>>>>>> is it intentional that rebooting a (HVM) guest after having altered its
>>>>>> vCPU count will reset it back to the vCPU count it was originally
>>>>>> started with? That doesn't seem very natural - if one hotplugs a CPU
>>>>>> into a physical system and then reboots, that CPU will remain there.
>>>>>>
>>>>> This is probably an oversight.
>>>>>
>>>>> I've added this to my list of things to look at after the release.
>>>> Thanks!
>>>>
>>> I got a patch ready.  But QEMU upstream refuses to start on the receiving 
>>> end
>>> with following error message:
>>>
>>> qemu-system-i386: Unknown savevm section or instance 'cpu_common' 1
>>> qemu-system-i386: load of migration failed: Invalid argument
>>>
>>> With QEMU traditional HVM guest and PV guest, the guest works fine -- up
>>> and running with all hot plugged cpus available.
>>>
>>> So I think the relevant libxl information is transmitted but we also
>>> need to fix QEMU upstream. But that's a separate issue.
>>>
>>> Wei.
>>>
>>> ---8<---
>>> From 790ff77c6307b341dec0b4cc5e2d394e42f82e7c Mon Sep 17 00:00:00 2001
>>> From: Wei Liu <wei.liu2@xxxxxxxxxx>
>>> Date: Fri, 3 Jun 2016 16:38:32 +0100
>>> Subject: [PATCH] libxl: update vcpus bitmap in retrieved geust config
>>>
>>> ... because the available vcpu bitmap can change during domain life time
>>> due to cpu hotplug and unplug.
>>>
>>> Reported-by: Jan Beulich <jbeulich@xxxxxxxx>
>>> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
>> This patch has two issues:
>> 1. The code to allocate bitmap is wrong
>> 2. The check to see if vcpu is available is wrong
>>
>> It happens to work on PV and qemu-trad because they don't really rely
>> on the bitmap provided.
>>
>> #1 is fixed.
>>
>> For #2, I haven't really gotten to the point today on how to correctly
>> get the number of online vcpus.
>>
>> The current issue I discover is that:
>>
>>   xl vcpu-set jessie-hvm 4
>>   xl list -l jessie-hvm | less # search for avail_vcpus
>>
>> A vcpu is not really considered online from xen's point of view, unless
>> the guest explicitly activates it, like in guest `echo 1 >
>> .../cpu1/online`.
>>
>> This is still not desirable because it would still cause qemu upstream
>> migration to fail. I will see if there is other way to figure out how
>> many vcpus are there.
>>
>> Off the top of my head I might need to interrogate QEMU for that. I will
>> continue investigation later.
>>
>> Any hint on how to effectively identify online vcpus is very welcomed.

Why does qemu even care?  It has nothing to do with vcpu handling. 
There should not be any qemu vcpu records in the first place.

~Andrew

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