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Re: [Xen-devel] [PATCH] [VTD] Remove PCI device enumeration for dom0 in Xen

Espen Skoglund wrote:
[Keir Fraser]
On 22/10/08 13:38, "Espen Skoglund" <espen.skoglund@xxxxxxxxxxxxx> wrote:

Xen 3.3 should continue to work with older dom0 kernels. However,
now that iommu is *disabled* by default, will this mean that it
would continue to work with older dom0 kernels so long as iommu is
not enabled on the Xen command line? This should be tested and
confirmed with this patch applied.  If so, I would accept it.
Users of pci_lock_pdev() and pci_lock_domain_pdev() would not be
able to find the PCI device unless it has been registered by dom0.
Apart from the iommu code, the only user affected would be the MSI
code which looks up the PCI device before enabling MSIs on it.  As
such, you would not be able to enable MSIs if you have an old dom0

Is there any disadvantage to keeping this legacy code in Xen, apart
from it annoyingly sitting there?

You would get a list of PCI devices in Xen containing entries that are
not necessarily in use.  Some of the entries might even be bogus.
Apart from some annoying extra code and additional entries in the
IOMMU page tables, I don't think there is much of a problem.  There
could be some potential problems if Xen at a future stage decides to
do some in-hypervisor cleverness with all known PCI devices (a not
very likely scenario IMHO).

We would have bus number rebalance (hotplug, etc.) and PCI domain support in the future, which may require hypervisor to keep consistent device list with dom0 kernel. If we don't remove the code, we still have to fix the list up.


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