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

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:

quoted

On Fri, Jan 04, 2013 at 06:07:51PM +0100, Daniel Kiper wrote:

quoted

On Fri, Jan 04, 2013 at 02:41:17PM +0000, Jan Beulich wrote:

quoted

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quoted

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On 04.01.13 at 15:22, Daniel Kiper [off-list ref] wrote:

On Wed, Jan 02, 2013 at 11:26:43AM +0000, Andrew Cooper wrote:

quoted

/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