Re: [Xen-devel] [PATCH v3 00/11] xen: Initial kexec/kdump implementation

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Mon, Jan 07, 2013 at 01:34:04PM +0100, Daniel Kiper wrote:
> On Fri, Jan 04, 2013 at 02:11:46PM -0500, Konrad Rzeszutek Wilk wrote:
> > On Fri, Jan 04, 2013 at 06:07:51PM +0100, Daniel Kiper wrote:
> > > On Fri, Jan 04, 2013 at 02:41:17PM +0000, Jan Beulich wrote:
> > > > >>> On 04.01.13 at 15:22, Daniel Kiper <daniel.kiper@oracle.com> wrote:
> > > > > On Wed, Jan 02, 2013 at 11:26:43AM +0000, Andrew Cooper wrote:
> > > > >> /sbin/kexec can load the "Xen" crash kernel itself by issuing
> > > > >> hypercalls using /dev/xen/privcmd.  This would remove the need for
> > > > >> the dom0 kernel to distinguish between loading a crash kernel for
> > > > >> itself and loading a kernel for Xen.
> > > > >>
> > > > >> Or is this just a silly idea complicating the matter?
> > > > >
> > > > > This is impossible with current Xen kexec/kdump interface.
> > > >
> > > > Why?
> > >
> > > Because current KEXEC_CMD_kexec_load does not load kernel
> > > image and other things into Xen memory. It means that it
> > > should live somewhere in dom0 Linux kernel memory.
> >
> > We could have a very simple hypercall which would have:
> >
> > struct fancy_new_hypercall {
> > 	xen_pfn_t payload; // IN
> > 	ssize_t len; // IN
> > #define DATA (1<<1)
> > #define DATA_EOF (1<<2)
> > #define DATA_KERNEL (1<<3)
> > #define DATA_RAMDISK (1<<4)
> > 	unsigned int flags; // IN
> > 	unsigned int status; // OUT
> > };
> >
> > which would in a loop just iterate over the payloads and
> > let the hypervisor stick it in the crashkernel space.
> >
> > This is all hand-waving of course. There probably would be a need
> > to figure out how much space you have in the reserved Xen's
> > 'crashkernel' memory region too.
> 
> I think that new kexec hypercall function should mimics kexec syscall.
> It means that all arguments passed to hypercall should have same types
> if it is possible or if it is not possible then conversion should be done
> in very easy way. Additionally, I think that one call of new hypercall
> load function should load all needed thinks in right place and
> return relevant status. Last but not least, new functionality should

We are not restricted to just _one_ hypercall. And this loading
thing could be similar to the micrcode hypercall - which just points
to a virtual address along with the length - and says 'load me'.

> be available through /dev/xen/privcmd or directly from kernel without
> bigger effort.

Perhaps we should have a email thread on xen-devel where we hash out
some ideas. Eric, would you be OK included on this - it would make
sense for this mechanism to be as future-proof as possible - and I am not
sure what your plans for kexec are in the future?
> 
> Daniel