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

[Yu Zhao <yu.zhao@uniscape.net>]

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

Thanks,
Yu