Date: Fri, 19 Jul 2013 12:14:49 -0700
From: Greg KH <gregkh@linuxfoundation.org>
To: Daniel Kiper <daniel.kiper@oracle.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Eric Biederman <ebiederm@xmission.com>, linux-kernel@vger.kernel.org,
        Brandon Philips <brandon@ifup.org>, xen-devel@lists.xensource.com,
        virtualization@lists.linux-foundation.org
Subject: Re: is kexec on Xen domU possible?
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On Fri, Jul 19, 2013 at 08:32:35PM +0200, Daniel Kiper wrote:
> On Fri, Jul 19, 2013 at 08:12:43AM -0700, Greg KH wrote:
> > On Fri, Jul 19, 2013 at 03:18:19PM +0200, Daniel Kiper wrote:
> > > > support is "easy" after your patches go in, is that because Dom0 needs
> > > > to support this, or is it something specific to only domU?
> > >
> > > In case of domU we should consider following cases:
> > >   - PV guests: there is no support for kexec at this time;
> > >     Once I wrote an implementatation for that type of guests
> > >     for one company but according to our agreement I could not
> > >     publish this code; However, I could use it as a base for
> > >     publicly available kexec implementation; Currently, I do
> > >     not have any plans to work on this due to some more important
> > >     stuff to do; However, question about kexec support for PV
> > >     guests is raised from time to time and maybe this issue
> > >     will be much more important than others once,
> > >   - HVM guests: kexec should work without any issue,
> > >   - PVonHVM guests: IIRC, there were some issues with PV
> > >     drivers but they were fixed some time ago by patches
> > >     posted by Olaf Hering,
> > >   - PVH guests: those type of guests are not available in Xen
> > >     current releases yet; However, Konrad Wilk done some preliminary
> > >     work on kexec support but there are still some issues to resolve.
> > >
> > > I do not know what are you trying to do but if you would like
> > > to get some crash dumps there is also another solution to that.
> > > You could use xm/xl dump-core from Dom0 to get dumps of domU memory.
> >
> > As Brandon said, we were trying to use kexec in a PV guest in domU to
> > run another kernel.  I had assumed this wouldn't need support from dom0.
> 
> You are right.
> 
> > As you have implemented this in the past, did you need to change dom0 in
> > order to achieve this, and if so, why?
> 
> It was strong requirement to not change anything in hypervisor or dom0.
> I succeeded to do that but it requires changes in kernel and kexec-tools.
> 
> > The errors that the kexec tools seem to run into is finding the memory
> > to place the new kernel into, is that just an issue that PV guests
> > aren't given enough kernel memory in which to replicate themselves from
> > dom0?
> 
> There are a lot of differences between baremetal machines and PV guests.
> For example you are not able to do identity mapping per se in PV guests.
> Arguments to new kernel are passed in completely different way. etc.

Ok, thanks for confirming that it is possible, but doesn't currently
work for pv guests.

greg k-h
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