Re: "xl vcpu-set" not persistent across reboot?

[Wei Liu <wei.liu2@citrix.com>]

On Mon, Jun 06, 2016 at 06:34:44PM +0100, Andrew Cooper wrote:
> 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@citrix.com> 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@citrix.com>
> >>> 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@suse.com>
> >>> Signed-off-by: Wei Liu <wei.liu2@citrix.com>
> >> 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.
> 

IIRC upstream rejected the idea of having no cpu attached to a platform.

Wei.

> ~Andrew

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