Re: [RFC] [PATCH 00/13] Introduce task_pid api

[ebiederm@xmission.com (Eric W. Biederman)]

Dave Hansen <haveblue@us.ibm.com> writes:

> On Wed, 2005-12-07 at 15:17 -0700, Eric W. Biederman wrote:
>> But beyond that a general test to see if you have done a good
>> job of virtualizing something is to see if you can recurse.
>
> I admit it would be interesting at the very least.  But, using that
> definition, we haven't done any good virtualization in Linux that I can
> think of.  Besides some vague ranting I heard about zSeries (the real
> IBM mainframes) I can't think of anything that does this today.
>
> I don think any of Solaris containers, ppc64 LPARs, Xen, UML, or
> vservers can recurse.  
>
> Can you think of any?

There is Xnest that allows X to run on X.

There are process groups and sessions that while they may
not strictly nest you don't loose the ability to create new
ones.

There is the CLONE_NEWNS and just about any of the other
clone flags in linux.

There is bochs that emulates the whole machine.

I am actually a little surprised that UML can't run UML.  I
suspect it is an address space conflict and not something fundamental.

With pidspaces as long as the parent isn't required to send
signals to arbitrary children  I don't think nesting pids spaces
is hard.  Or more properly have a process in one pidspace be
the parent of a process in another.  Although I grant there
are a few boundary issues, that have to be handled carefully.

Eric