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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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.

Wei.

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