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

[Yuji Shimada <shimada-yxb@xxxxxxxxxxxxxxx>]

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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