Re: [Xen-devel] [PATCH] xen/pci: make bus notifier handler return sane values

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 25.08.11 at 11:14, George Dunlap <george.dunlap@xxxxxxxxxx> wrote:
> On Wed, 2011-08-24 at 21:42 +0100, Konrad Rzeszutek Wilk wrote:
>> > Requiring the PCI device to be known to Xen to be able to use MSI is
>> > an implementation detail of the hypervisor. And in fact I had a patch
>> > to remove the call to pci_get_pdev() from map_domain_pirq() (since
>> > setup_msi_irq() doesn't really consume this parameter). This got made
>> > impossible to do (at least for the moment) with George's
>> > 23753:2e0cf9428554 (and later on the device is getting looked up by
>> > __pci_enable_msi{,x}() anyway, but even that is an implementation
>> > choice, not a hard requirement afaict).
> 
> The device has to be known to Xen in order to do proper interrupt
> remapping, does it not?  At least if the AMD per-device interrupt
> remapping tables.

Yes, but doesn't this only affect device passthrough? Further, Xen
does its own (poor man's) PCI bus scan, so generally it knows of all
devices (at least on segment 0).

Additionally the Dom0 kernel doesn't even know whether IOMMUs are
actually in use by Xen.

Bottom line - imo still no reason to disable MSI (neither globally nor for
the device).

Jan

> I suppose if you really wanted to, we could refactor the code so that
> the time Xen needed to know devices for IRQs coincided exactly with when
> Xen needs to know devices for IOMMUs.  I don't understand the motivation
> for such a requirement yet.
> 
>  -George




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