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

On Thu, 2011-07-07 at 10:10 +0100, Jan Beulich wrote:
> >>> 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.

That's arguably a debugging facility as well though.

> 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.

The problem is that this can't be done without device specific
knowledge, which the hypervisor generally doesn't have and we can't get
the device's owning domain to do anything because we are crashing.


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