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Re: [Xen-devel] [PATCH] dom0 linux: Reassign memory resources to device for pci passthrough.

On Thu, 9 Oct 2008 11:21:22 +0800
"Zhao, Yu" <yu.zhao@xxxxxxxxx> wrote:

> On Thursday, October 09, 2008 7:44 AM, Yuji Shimada wrote:
> >There is a similar function enabled by pci-mem-align boot
> >parameter. Currently it is kept. But if many people agree with me, I'd
> >like to remove it from dom0 linux, because there are two problems.
> >
> >    - pci-mem-align reassigns all devices' memory resources if they are
> >      not page-aligned. This is not safe, because some devices are
> >      used by firmware.
> >    - pci-mem-align can't expand resource window of PCI-PCI bridge.
> Why the resource windows can't be expended? I think the
> pci_bus_size_bridges takes care of this. So it shouldn't be a
> problem and your pci_update_bridge appears unnecessary.

Current pci_bus_size_bridges calculates the size and minimal alignment
of resource window based on actual resource size. The values of them
will be not changed if pci-mem-align is enabled.

But if we make memory resource page-aligned, we need more space. My
patch rounds up resource size to page size, on calculating the size
of resource windows.

> Regarding the first problem you mentioned, it could be easily fixed
> by merging your "rassigndev=" parameter to quirk_align_mem_resources
> so the function can selective twist the devices.

If my "rassigndev=" parameter is merged to quirk_align_mem_resources,
problem will occurs. Current quirk_align_mem_resources shifts
resources simply. Resources will conflict easily. If confliction
occurs, kernel reassigns conflicting resources. As a result, the
resources of device which are not specified are reassigned.
Additionally, the reassigned resource is not page-aligned.

Yuji Shimada

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