Re: [Xen-devel] [PATCH] PCI / ACPI: Do not set ACPI companions for host bridges with parents

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 05/26/2015 08:07 PM, Rafael J. Wysocki wrote:
> On Tuesday, May 26, 2015 10:32:03 AM Boris Ostrovsky wrote:
> > On 05/25/2015 10:17 PM, Rafael J. Wysocki wrote:
> > > On Tuesday, May 26, 2015 03:08:17 AM Rafael J. Wysocki wrote:
> > > > On Tuesday, May 26, 2015 01:42:16 AM Rafael J. Wysocki wrote:
> > > > > 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().
> > > > And here's one more thing to test.
> > > And the below is how I'd fix it, so you can simply test this patch and skip the
> > > previous ones.
> > >
> > > ---
> > > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > > Subject: PCI / ACPI: Do not set ACPI companions for host bridges with parents
> > >
> > > Commit 97badf873ab6 (device property: Make it possible to use
> > > secondary firmware nodes) uncovered a bug in the x86 (and ia64) PCI
> > > host bridge initialization code that assumes bridge->bus->sysdata
> > > to always point to a struct pci_sysdata object which need not be
> > > the case (in particular, the Xen PCI frontend driver sets it to point
> > > to a different data type).  If it is not the case, an incorrect
> > > pointer (or a piece of data that is not a pointer at all) will be
> > > passed to ACPI_COMPANION_SET() and that may cause interesting
> > > breakage to happen going forward.
> > >
> > > To work around this problem use the observation that the ACPI
> > > host bridge initialization always passes NULL as parent to
> > > pci_create_root_bus(), so if pcibios_root_bridge_prepare() sees
> > > a non-NULL parent of the bridge, it should not attempt to set
> > > an ACPI companion for it, because that means that
> > > pci_create_root_bus() has been called by someone else.
> > I am wondering whether at some point we might want to use companion
> > information in Xen as well.
> >
> > Another option might be to require that whoever creates their own
> > sysdata structure to have pci_sysdata as its first element, e.g.
> Well, I was thinking about that, but it would be x86-specific and I believe
> that Xen is supposed to work on other architectures too (or at least will be
> in the future).
>
> > --- a/drivers/pci/xen-pcifront.c
> > +++ b/drivers/pci/xen-pcifront.c
> > @@ -52,7 +52,12 @@ struct pcifront_device {
> >    };
> >
> >    struct pcifront_sd {
> > +       /*
> > +        * Must be the first element of this structure since pcifront_sd
> > +        * is assigned to bus' sysdata and may later be dereferenced as
> > +        * pci_sysdata.
> > +        */
> > +       struct pci_sysdata sd;
> >           struct pcifront_device *pdev;
> >    };
> Also if you want to use an ACPI companion, it will be a good question *which* 
> one.
>
> My half-way educated guess is that will be the ACPI companion of the parent,
> in which case it's better to modify pcibios_root_bridge_prepare().  In any
> case, we need to talk about that beforehand, so please let me know when you
> have more specific plans.
I don't have anything specific. And after looking at this code some more 
I am not sure pcifront will ever want to use companions since ACPI is 
not part of device initialization here (it's done via xenbus, which is a 
purely software construct).

> As for 4.1 (which currently is broken for Sander), I think the $subject patch
> is the cleanest and least intrusive thing we can do.
OK. Thank you for fixing this.

-boris


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