Re: [Qemu-devel] [patch 2/2] QEMU BOCHS bios patches to use maxcpus value.

[Anthony Liguori]

Avi Kivity wrote:
> On 07/10/2009 12:57 AM, Anthony Liguori wrote:
> > These changes make my Ubuntu server guest very unhappy.  I get a 
> > bunch of messages about "Not responding." on startup.
> >
> > If nothing else, maxcpus ==smp_cpus under QEMU because we don't do 
> > CPU hotplug (and I don't think we should).
>
> Why shouldn't we do cpu hotplug?

I don't think we should do cpu hotplug via ACPI.  I don't think ACPI 
actually models CPU hotplug and the fact that this works with Linux in 
KVM is a happy accident.  VMware only supports CPU hotplug for Windows 
7/2k8 guests so I'm assuming their using Viridian PV extensions to do it.

I think we should go the PV route for Linux too.  I'd rather see us 
create all vcpu threads at once and then let the guest offline each vcpu 
via a PV notification.  I don't see a lot of value in 
spawning/terminating vcpu threads dynamically and it adds an awful lot 
of complexity.  There's very little overhead to an idle thread.

Regards,

Anthony Liguori