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:

quoted

On Fri, Jul 19, 2013 at 03:18:19PM +0200, Daniel Kiper wrote:

quoted

quoted

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.

quoted

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.

quoted

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