libvirt and nestedhvm.

libvirt and nestedhvm.

[Alvin Starr]

I am trying to shoehorn the ability to start a nested HVM into libvirt 
through the libxl interface.

Setting the nestedhvm flag is easy enough but I also need to mask the 
svm_npt cpuid flags.

This is where life gets interesting.

LIbvirt seems to probe the CPUID registers to determine what flags are 
available.
Libvirt then stops me from trying to mask the svm or npt flags since 
they are not visible in dom0.

So my question is.
Can I enable the svm,npt flags in dom0?
Is there anything equivalent to nestedhvm for dom0?

or is there a way to read the raw CPU flags?



-- 
Alvin Starr                   ||   voice: (905)513-7688
Netvel Inc.                   ||   Cell:  (416)806-0133
alvin@netvel.net              ||

Re: libvirt and nestedhvm.

[Andrew Cooper]

On 06/06/14 14:28, Alvin Starr wrote:
> I am trying to shoehorn the ability to start a nested HVM into libvirt
> through the libxl interface.
>
> Setting the nestedhvm flag is easy enough but I also need to mask the
> svm_npt cpuid flags.

This presumably comes from a KVM centric view where the environment
running libvirtd is the host environment?

>
> This is where life gets interesting.
>
> LIbvirt seems to probe the CPUID registers to determine what flags are
> available.
> Libvirt then stops me from trying to mask the svm or npt flags since
> they are not visible in dom0.
>
> So my question is.
> Can I enable the svm,npt flags in dom0?

Technically it is possible to make these flags appear...

> Is there anything equivalent to nestedhvm for dom0?

... but being features that PV guests cannot possibly use, advertising
them is a bad idea.

>
> or is there a way to read the raw CPU flags?
>
Not really.

It is one of the many things on my bucketlist of basic things which need
fixing with all the other Xen/libxc/toolstack cpuid work.

~Andrew