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

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Date:	Wed, 27 May 2015 02:07:16 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	Sander Eikelenboom <linux@...elenboom.it>, david.vrabel@...rix.com,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux PCI <linux-pci@...r.kernel.org>
Subject: Re: [PATCH] PCI / ACPI: Do not set ACPI companions for host bridges with parents

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@...el.com>
> > 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.

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.

Thanks,
Rafael

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