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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Jun 07, 2016 at 12:38:26AM -0600, Jan Beulich wrote:
> >>> On 06.06.16 at 19:18, <wei.liu2@xxxxxxxxxx> wrote:
> > 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.
> 
> How come xl itself doesn't know right from the "xl vcpu-set"?
> 

That is because:

1. libxl only pulls data from various sources when it needs to right
   at the moment you ask for domain configuration.
2. For QEMU upstream, there is no way in telling whether a hotplug is
   successful or not -- the return value is ignored because response
   from QEMU is not sensible.

Not saying these things are completely unfixable though.

Wei.

> And considering the guest reboot case I originally had the issue
> with, the guest not activating the vCPU while still running would
> not mean it won't use it post reboot. E.g. if the guest OS is not
> CPU-hotplug capable, the reboot may be for the very reason of
> activating the extra vCPU(s)...
> 
> Jan
> 

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