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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On Thu, 2011-08-25 at 11:07 +0100, Jan Beulich wrote:
> >>> 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).

>From what I understand, the IOMMU doesn't know which devices are being
passed through; if it's on, all devices have to be given permissions for
whatever they need to do.  You can boot with iommu=off, but then you
can't safely pass any devices through to other VMs, including driver
domains, and you can't pass devices through to HVM domains at all (no
address translation for DMA).

> 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).

I have no idea about this.  I was just commenting about my c/s requiring
Xen to know about PCI devices (which is I assume why you cc'd me). :-)

 -George


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