Re: [PATCH 11/11] xen/hvm kdump: reset PV devices in crash kernel

[Olaf Hering <olaf@aepfle.de>]

On Fri, Jul 29, Stefano Stabellini wrote:

> On Thu, 28 Jul 2011, Olaf Hering wrote:
> > (this is actually a forward port of rev 1079 from 2.6.18.hg, untested because
> > kdump just hangs before (or while) entering the crash kernel in 3.0)
> > 
> > After triggering a crash dump in a HVM guest, the PV backend drivers
> > will remain in connected state. When the kdump kernel starts the PV
> > drivers will skip such devices. As a result, no root device is found and
> > the vmcore cant be saved.
> > 
> > With this change all frontend devices with state XenbusStateConnected
> > will be reset by changing the state file to Closing/Closed/Initializing.
> > This will trigger a disconnect in the backend drivers. Now the frontend
> > drivers will find the backend drivers in state Initwait and can connect.
> > 
> > Signed-off-by: Olaf Hering <olaf@aepfle.de>
> > 
> > ---
> >  drivers/xen/xenbus/xenbus_comms.c          |    4 -
> >  drivers/xen/xenbus/xenbus_probe_frontend.c |   97 +++++++++++++++++++++++++++++
> >  2 files changed, 100 insertions(+), 1 deletion(-)
> > 
> > Index: linux-3.0/drivers/xen/xenbus/xenbus_comms.c
> > ===================================================================
> > --- linux-3.0.orig/drivers/xen/xenbus/xenbus_comms.c
> > +++ linux-3.0/drivers/xen/xenbus/xenbus_comms.c
> > @@ -212,7 +212,9 @@ int xb_init_comms(void)
> >  		printk(KERN_WARNING "XENBUS response ring is not quiescent "
> >  		       "(%08x:%08x): fixing up\n",
> >  		       intf->rsp_cons, intf->rsp_prod);
> > -		intf->rsp_cons = intf->rsp_prod;
> > +		/* breaks kdump */
> > +		if (!reset_devices)
> > +			intf->rsp_cons = intf->rsp_prod;
> >  	}
> 
> Where is reset_devices coming from?
> I hope is set only on a kexec reboot.

This is for a kdump boot, its added as additional kernel cmdline option
for the kdump kernel.

> Considering that all the other patches in this series follow the
> opposite strategy, that is closing stuff on shutdown, why are you trying
> to close xenbus connections on boot here?
> At this point I would rather be coherent and simply switch to
> XenbusStateClosing or XenbusStateClosed in the shutdown path if
> kexec_is_loaded.

To allow the kdump kernel to store the /proc/vmcore file somewhere, it
has to get either storage or network access.  But the kdump kernel finds
all devices in the Connected state, because the to-be-debugged kernel
just crashed. So it has to reset the connection to the backend.

Olaf