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[Xen-devel] [PATCH] Add "Assignable device to HVM domain" section to vtd.txt

This patch adds "Assignable device to HVM domain" section to vtd.txt.

Some devices are not designed to be assigned to HVM domain
friendly. Currently, there is not a good idea to make them work, as
discussed last week.

For now, I think some information about this should be available in
document to share knowledge among developers and users.

diff -r dbac9ee4d761 docs/misc/vtd.txt
--- a/docs/misc/vtd.txt Mon Sep 08 16:02:13 2008 +0100
+++ b/docs/misc/vtd.txt Tue Sep 09 08:57:34 2008 +0900
@@ -1,8 +1,9 @@ Title   : How to do PCI Passthrough with
 Title   : How to do PCI Passthrough with VT-d
 Authors : Allen Kay    <allen.m.kay@xxxxxxxxx>
           Weidong Han  <weidong.han@xxxxxxxxx>
+          Yuji Shimada <shimada-yxb@xxxxxxxxxxxxxxx>
 Created : October-24-2007
-Updated : August-06-2008
+Updated : September-09-2008
 How to turn on VT-d in Xen
@@ -106,3 +107,27 @@ http://h10010.www1.hp.com/wwpc/us/en/en/
 For more information, pls refer to http://wiki.xensource.com/xenwiki/VTdHowTo.
+Assignable device to HVM domain
+Most of the devices like NIC, HBA, EHCI and UHCI can be assigned to
+HVM domain.
+But some devices are not designed to be assigned to HVM domain
+friendly. One of unfriendly designs is as follows.
+ * Device has a internal resource like private memory which is
+   mapped to memory address space with BAR (Base Address Register).
+ * Driver submits command with a pointer to a buffer within internal
+   resource. Device decodes the pointer (address), and accesses to the
+   buffer.
+In HVM domain, BAR is virtualized, and host-BAR value and guest-BAR
+value are different. The addresses of internal resource from device's
+view and driver's view are different. Similarly, the addresses of
+buffer within internal resource from device's view and driver's view
+are different. As a result, device can't access to the buffer
+specified by driver.
+Such devices assigned to HVM domain don't work.

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