[Xen-changelog] [xen-unstable] vtd.txt: Add caveat about assigning certain types of device.

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1220968496 -3600
# Node ID 1f89af507e6235ff9276e32f02f5de83eaef5a6a
# Parent  5ce9459ce8722a8af89da5a73b0c80a767d5b1ad
vtd.txt: Add caveat about assigning certain types of device.

From: Yuji Shimada <shimada-yxb@xxxxxxxxxxxxxxx>
Signed-off-by: Keir Fraser <keir.fraser@xxxxxxxxxx>
---
 docs/misc/vtd.txt |   27 ++++++++++++++++++++++++++-
 1 files changed, 26 insertions(+), 1 deletion(-)

diff -r 5ce9459ce872 -r 1f89af507e62 docs/misc/vtd.txt
--- a/docs/misc/vtd.txt Tue Sep 09 14:50:29 2008 +0100
+++ b/docs/misc/vtd.txt Tue Sep 09 14:54:56 2008 +0100
@@ -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.
 
+
+Assigning devices to HVM domains
+--------------------------------
+
+Most device types such as NIC, HBA, EHCI and UHCI can be assigned to
+an HVM domain.
+
+But some devices have design features which make them unsuitable for
+assignment to an HVM domain. Examples include:
+
+ * Device has an internal resource, such as 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 an HVM domain, the 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 currently do not work.

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