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

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Thu, 2011-07-07 at 10:42 +0100, Jan Beulich wrote:
> >>> On 07.07.11 at 11:12, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx> wrote:
> > 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.
> 
> Why would there be any device specific knowledge needed? It's
> all done through the command word, just that writing zero isn't
> really appropriate.

So presumably if you disable bus mastering you've effectively disabled
DMA but how do you disable interrupts via the command word?

Ian.


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