[Xen-devel] Re: xenstore index reset in xb_init_comms breaks domU kdump

[Keir Fraser <keir@xxxxxxx>]

On 26/01/2011 14:00, "Olaf Hering" <olaf@xxxxxxxxx> wrote:

> 
> Keir,
> 
> in the commit below a reset of the rsp_cons/rsp_prod index was added.
> http://xenbits.xen.org/staging/xen-unstable.hg?rev/1ec0d322402e
> 
> Is this reset really required for save/restore? I have not tried it, but
> I would assume a restored guest would get an empty page to work with.

No, I'm pretty sure it doesn't get a fresh page on restore.

> Also:
> Could a guest be saved when its in the middle of a xenstore operation?

No, a pv (or pv-on-hvm) guest suspends itself, and can therefore quiesce its
xenstore subsystem.

> The reset breaks the xenstore interface in a crashed domU kernel. If the
> crash kernel boots, it tries to operate on the new rsp_* index. But this
> contains garbage, process_msg() tries to use the data as 'struct
> xsd_sockmsg', and kmalloc() fails.
> If I remove the rsp_cons/rsp_prod reset in xb_init_comms(), the PV
> drivers in the crash kernel can connect to xenstore.
> 
> Should the fixup code be removed, or is it essential for save/restore?

I think it is needed for save/restore. It may not be needed during boot
though. Your safest (lowest impact) fix would be to skip the reset when
booting a crash kernel. A kernel knows it is a crash kernel, I believe?

 -- Keir

> 
> Olaf
> 



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