[vbox-dev] Can't use more than one vCPU after upgrading toVirtualBox 4.3.x

Frank Mehnert frank.mehnert at oracle.com
Sat Dec 28 16:51:51 GMT 2013

Hi Tal,

as I thought: You have a newer AMD processor and Windows returns the
wrong number of cores for your CPU: It returns 1 when your CPU has 4
physical cores. We use a Windows system function to determine the number
of physical cores. I think that's a Windows bug and we need to find
a workaround. To be able to assign more guest CPUs to your VMs you
can use 'VBoxManage modifyvm VM_NAME --cpus ...'.

Kind regards,


On Friday 27 December 2013 16:20:29 Tal Aloni wrote:
> Thanks Frank,
> I've attached the two log files (from 4.2.20 and 4.3.6).
> Tal
> ----- Original Message -----
> From: "Frank Mehnert" <frank.mehnert at oracle.com>
> To: <vbox-dev at virtualbox.org>
> Sent: Friday, December 27, 2013 4:28 PM
> Subject: Re: [vbox-dev] Can't use more than one vCPU after upgrading
> toVirtualBox 4.3.x
> Hi Tal,
> On Thursday 26 December 2013 16:52:34 Tal Aloni wrote:
> > I have an AMD A8-3850 CPU, which have 4 cores.
> > I noticed that v4.3.0 introduced some changes to the CPU core detection
> > mechanism, unfortunately, it does not work too well for my CPU, and I can
> > no longer present more than 1 vCPU to each virtual machine (with 4.2.x I
> > could assign up to 4).
> > 
> > I'll be glad to provide additional information and tests beta builds if
> > necessary, Thanks,
> indeed, VBox 4.3.x changed the detection of the number of CPU cores. Could
> you provide me a VBox.log file of a VM session? That log file will provide
> some more information about your host CPU.
> Thanks,
> Frank

Dr.-Ing. Frank Mehnert | Software Development Director, VirtualBox
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