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

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 07.07.11 at 11:53, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx> wrote:
> 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?

Interrupts can be disabled equally well in the IO-APIC and by
disabling MSI for all devices.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel