a question about powervm

[Liu ping fan]

On Mon, Dec 19, 2016 at 5:58 PM, Michal Suchanek <hramrach at gmail.com> wrote:
> Hello,
>
> On 19 December 2016 at 10:09, Liu ping fan <kernelfans at gmail.com> wrote:
>> Hi guys,
>>
>> I am using a pSeries. It runs on powerVM.
>> My question is whether the cpuX under /sys/devices/system/cpu
>> corresponds to a real cpu or not.
>> I think it is not the same as the cpu on kvm-guest, which is emulated
>> by a linux process.
>
> In general the hypervisor (powerVM or KVM) needs to disable access of
> the guest to some CPU features in order to manage multiple guests. So
> some CPU features are either disabled or emulated by the hypervisor to
> prevent the guest from interfering with isolation of the guests from
> each other and the host. On the other hand, emulating the whole CPU is
> ineffective so to achieve reasonable guest speed it must run on a real
> CPU most of the time. That means that in most cases the CPU you see in
> the guest will have a subset of the features of the CPU the host sees.
>
Thank you for the explanation. And sorry not to explain my question clearly.
I wonders about the mapping between guest-CPU and physical cpu. On
powerVM, except the feature exposed by host, it is 1:1 map right?

> Some virtualization solutions allow to overcommit CPU time so you can
> allocate more CPUs to guests than you physically have. Then when
> running the guest will use an actual physical CPU but when it is
> sleeping other guest may use the CPU.
>
Yes, agree.

Thanks!