Re: [Xen-devel] Question about MMIO Region Alignment for PCI Pass-through

[Yosuke Iwamatsu <y-iwamatsu@xxxxxxxxxxxxx>]

Hi Barak,
Thank you for your reply.

Barak Fargoun wrote:
> Hi!
>
> I have created the 'pci-mem-align' patch. The version posted before, had
> some bugs. However, I have fixed those issues, and it works fine for me
> now.
What kinds of bugs did you fix?
It would be nice if I could see your new patch.

> BTW, why are you interested in this?
Because I'm recently working a bit on PV PCI passthrough.
I haven't encountered the mmio region conflict yet, but I think it is a
a potential problem for all the passthrough methods
(VT-d/IOMMU, PV driver domain and nativedom).

Regards,
------------------
  Yosuke Iwamatsu

> Barak
>
> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Yosuke
> Iwamatsu
> Sent: Wednesday, March 05, 2008 11:34 AM
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] Question about MMIO Region Alignment for
> PCIPass-through
>
> Hi there,
>
> I have a question about the mmio region access of PCI pass-trough
> devices.
>
> When assigning a PCI device to a guest, pages that contain the device
> mmio regions are mapped to the guest. Then the guest can directly issue
> mmio instructions. However, there is a problem that, if a single page
> has mmio regions of multiple devices, we cannot expose it to the guest.
> So we need to align PCI memory regions to page size (4K) in dom0 kernel
> and make sure that each page has only one device's mmio region.
>
> I found that this problem was discussed a bit in the following thread.
> http://lists.xensource.com/archives/html/xen-devel/2007-10/msg01012.html
> (linux-2.6.18-xen.hg: changest 296)
> Dom0 parameter "pci-mem-align" seems to help aligning mmio regions.
> But, at a quick look, mmio regions appear to be aligned not to PAGE_SIZE
> but to PAGE_SIZE-1. I also have a concern that re-mapping all the PCI
> resources might do harm to some devices unrelated to PCI pass-through.
>
> Does anyone have an idea about this mmio re-mapping issue ?
> (or is anyone already working on this ?)
>
> Regards,
> -------------------
> Yosuke Iwamatsu
>         NEC Corporation
>
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> http://lists.xensource.com/xen-devel

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