Re: [Xen-devel] [PATCH] KEXEC: disconnect all PCI devices from the PCI bus on crash

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 07.07.11 at 10:53, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Wed, 2011-07-06 at 19:42 +0100, Konrad Rzeszutek Wilk wrote:
>> On Wed, Jul 06, 2011 at 01:39:12PM +0100, Andrew Cooper wrote:
>> > +/* Disconnect a PCI device from the PCI bus.  From the PCI spec:
>> > + *     "When a 0 is written to [the COMMAND] register, the device is
>> > + *     logically disconnected from the PCI bus for all accesses except
>> > + *     configuration accesses. All devices are required to support
>> > + *     this base level of functionality."
>> > + */
>> > +void disconnect_pci_device(struct pci_dev *pdev)
>> > +{
>> > +    pci_conf_write16(pdev->bus, PCI_SLOT(pdev->devfn),
>> > +                     PCI_FUNC(pdev->devfn), PCI_COMMAND, 0);
>> 
>> So if you have a PCI serial card (or Intel AMT) and you are using that for
>> serial output on the hypervisor line, this will turn it off. There should
>> be some whitelist capability to not do it for PCI serial devices that are
>> owned (used) by the hypervisor.
> 
> That would be useful for debugging the kexec process itself but in the
> general case there won't be any further output from the hypervisor and
> if the kexec'd kernel wants to use the device it is going to have to set
> it up again anyways.

No, not generally. Just look at Linux' early-printk code: The device
is assumed to be enabled (by the BIOS), as the PCI subsystem can't
possibly be initialized at this point already.

This also means that white-listing just devices Xen uses may not be
enough: If Xen doesn't use a serial console (or the secondary kernel
wants to use some other device Xen doesn't care about - VGA or
other kind of console devices come to mind), it must not find it fully
disconnected from the bus. Consequently I would think that while
interrupt and DMA activity should be forced off, decoding I/O and
memory addresses by the devices shouldn't be.

Jan


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