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 11:02 +0100, Andrew Cooper wrote:
> 
> On 07/07/11 10:53, Ian Campbell 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?
> >
> > Ian.
> >
> Bit 10 of the control word is "disable assertion of INTx# pins"  (so set
> bit to 1 to disable interrupts).

My copy of the spec isn't new enough to cover that particular bit but
the Linux headers describe it as disabling INTx "emulation" so I wasn't
sure if it actually did anything in all cases, especially on older
hardware.

>   That should cover legacy interrupts. 
> For MSI and above, disabling DMA should prevent the bus writing to the
> magic local APIC addresses.
> 



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