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

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

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.
> 
> Wei.

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