Re: [Xen-devel] Regression due to "device property: Make it possible to use secondary firmware nodes" Re: Xen-unstable + linux 4.1-mergewindow: problems with PV guest pci passthrough: pcifront pci-0: pciback not responding!!!

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Tuesday, May 26, 2015 01:22:12 AM Rafael J. Wysocki wrote:
> On Friday, May 22, 2015 09:53:37 PM Boris Ostrovsky wrote:
> > On 05/22/2015 04:11 AM, Sander Eikelenboom wrote:
> > > Hello Sander,
> > >

[cut]

> > (+Rafael again)
> > 
> > So the immediate cause of those errors is that pdev->evtchn is 0. 
> > Backend is not notified and things not go well then.
> > 
> > And it is indeed caused by 97badf873ab60e841243b66133ff9eff2a46ef29:
> > 
> > We allocate pcifront_sd in pcifront_scan_root() and then pass it to 
> > pci_scan_bus_parented() as sysdata. Eventually this sysdata is used in 
> > pcibios_root_bridge_prepare() as pci_sysdata. It is dereferenced as 
> > pci_sysdata->companion (which I believe is aliased to pcifront_sd->pdev)

Well, there is an int node field between them, so I'm not sure.

> > and then set_primary_fwnode() writes it, thus corrupting 
> > pcifront_sd->pdev (and I think this is what sets evtchn to zero).

So the corruption happens when set_primary_fwnode() writes NULL to the
'secondary' field of object pointed to by 'fwnode'.

This isn't strictly necessary and we might avoid the crash by only
writing to fwnode->secondary if fn is not NULL.

So, Sander please test the patch below too if possible.

Of course, that doesn't solve a problem of passing an incorrect pointer
to ACPI_COMPANION_SET() in pcibios_root_bridge_prepare().

---
 drivers/base/core.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Index: linux-pm/drivers/base/core.c
===================================================================
--- linux-pm.orig/drivers/base/core.c
+++ linux-pm/drivers/base/core.c
@@ -2167,7 +2167,9 @@ void set_primary_fwnode(struct device *d
 		if (fwnode_is_primary(fn))
 			fn = fn->secondary;
 
-		fwnode->secondary = fn;
+		if (fn)
+			fwnode->secondary = fn;
+
 		dev->fwnode = fwnode;
 	} else {
 		dev->fwnode = fwnode_is_primary(dev->fwnode) ?