simple check for domU?

simple check for domU?

[Chris Bainbridge]

Hi, is there a simple way to check that we're running under a domU? So
far I came up with:

[[ -d /proc/xen && ! -z `ls /sys/bus/xen/devices/` ]]

or

-e /sys/bus/xen/drivers/vbd || -e /sys/bus/xen/drivers/vif

Neither of which is pretty, and I'm not convinced they work under all
circumstances. This is useful for system setup scripts to do different
things depending if they are run under dom0/U.

Thanks,
Chris

Re: simple check for domU?

[Karsten M. Self]

on Fri, Nov 11, 2005 at 12:23:18AM +0000, Chris Bainbridge (chris.bainbridge@gmail.com) wrote:
> Hi, is there a simple way to check that we're running under a domU? So
> far I came up with:

Try:

    uname -r | grep -q -- -xen

... which generally works.  'test -d /proc/xen' is pretty reliable as
well.
 
> [[ -d /proc/xen && ! -z `ls /sys/bus/xen/devices/` ]]
> 
> or
> 
> -e /sys/bus/xen/drivers/vbd || -e /sys/bus/xen/drivers/vif
> 
> Neither of which is pretty, and I'm not convinced they work under all
> circumstances. This is useful for system setup scripts to do different
> things depending if they are run under dom0/U.
> 
> Thanks,
> Chris
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

-- 
Karsten M. Self <karsten@xensource.com>
XenSource, Inc.
2300 Geng Road #250                                +1 650.798.5900 x259
Palo Alto, CA 94303                                +1 650.493.1579 fax

Re: simple check for domU?

[Chris Bainbridge]

On 11/11/05, Karsten M. Self <kself@xensource.com> wrote:
> on Fri, Nov 11, 2005 at 12:23:18AM +0000, Chris Bainbridge (chris.bainbridge@gmail.com) wrote:
> > Hi, is there a simple way to check that we're running under a domU? So
> > far I came up with:
>
> Try:
>
>     uname -r | grep -q -- -xen
>
> ... which generally works.  'test -d /proc/xen' is pretty reliable as
> well.

I'm looking specifically for domU and not dom0 (well, it would be
useful to have a reliable way of identifying both). /proc/xen will be
present in dom0 and uname depends on the kernel EXTRAVERSION which
isn't guaranteed.

Re: simple check for domU?

[NAHieu]

On 11/11/05, Chris Bainbridge <chris.bainbridge@gmail.com> wrote:
> Hi, is there a simple way to check that we're running under a domU? So
> far I came up with:
>
> [[ -d /proc/xen && ! -z `ls /sys/bus/xen/devices/` ]]
>
> or
>
> -e /sys/bus/xen/drivers/vbd || -e /sys/bus/xen/drivers/vif
>
> Neither of which is pretty, and I'm not convinced they work under all
> circumstances. This is useful for system setup scripts to do different
> things depending if they are run under dom0/U.

That is easy. Something you will never see in domU, such as
xenconsoled, xenstored,...

So check for the presence of these daemons is a good way, especially
that requires no privileged access.

Re: simple check for domU?

[Chris Bainbridge]

On 11/11/05, NAHieu <nahieu@gmail.com> wrote:
> On 11/11/05, Chris Bainbridge <chris.bainbridge@gmail.com> wrote:
> > Hi, is there a simple way to check that we're running under a domU? So
> > far I came up with:
> >
> > [[ -d /proc/xen && ! -z `ls /sys/bus/xen/devices/` ]]
> >
> > or
> >
> > -e /sys/bus/xen/drivers/vbd || -e /sys/bus/xen/drivers/vif
> >
> > Neither of which is pretty, and I'm not convinced they work under all
> > circumstances. This is useful for system setup scripts to do different
> > things depending if they are run under dom0/U.
>
> That is easy. Something you will never see in domU, such as
> xenconsoled, xenstored,...
>
> So check for the presence of these daemons is a good way, especially
> that requires no privileged access.

It is quite possible that someone has these daemons installed in a
domU fs. Maybe you mean check if the process is running? Even that is
problematic; it's possible to run a xen0 kernel without those daemons.
Besides, how do we differentiate between a domU kernel and a standard
kernel?

If there's no easy and reliable way to do this maybe one should be added?

RE: simple check for domU?

[James Harper]

> 
> If there's no easy and reliable way to do this maybe one should be
added?
> 

You probably need to clarify exactly what you are asking. If you are
running a -xen kernel instead of -xen[0U], then it's the same kernel but
differing in what privileges you have. In theory (I hope I'm right when
I say this), it is possible to give a domU kernel some access to
hardware, so even the line between a privileged or not kernel is a
little blurred. 

If you had CONFIG_ICONFIG set in your .config, you could grep for
various config options in proc, which would tell you if the kernel was
compiled with backend or frontend support, but that still doesn't really
help much.

James

Re: simple check for domU?

[Chris Bainbridge]

On 12/11/05, James Harper <james.harper@bendigoit.com.au> wrote:
> >
> > If there's no easy and reliable way to do this maybe one should be
> added?
> >
>
> You probably need to clarify exactly what you are asking. If you are
> running a -xen kernel instead of -xen[0U], then it's the same kernel but
> differing in what privileges you have. In theory (I hope I'm right when
> I say this), it is possible to give a domU kernel some access to
> hardware, so even the line between a privileged or not kernel is a
> little blurred.

The Gentoo init scripts do some stuff that fails under a domU
(hwclock, ntp..). There's a function that tries to detect a if the
kernel is a xenU so as to not to do the things that fail, but it's
currently broken (http://bugs.gentoo.org/show_bug.cgi?id=107976). It
would be nice to have a simple way to fix it.