[Xen-changelog] [xen-unstable] docs: add a document about guest cpuid configuration

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

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1260777580 0
# Node ID e73d666b4d3016a83a65fbe392a9600895aad161
# Parent  7088591b454bb6ed02d2f1ddd4947ced29e6ab4f
docs: add a document about guest cpuid configuration

Signed-off-by: Dexuan Cui <dexuan.cui@xxxxxxxxx>
---
 docs/misc/cpuid-config-for-guest.txt |   23 +++++++++++++++++++++++
 1 files changed, 23 insertions(+)

diff -r 7088591b454b -r e73d666b4d30 docs/misc/cpuid-config-for-guest.txt
--- /dev/null   Thu Jan 01 00:00:00 1970 +0000
+++ b/docs/misc/cpuid-config-for-guest.txt      Mon Dec 14 07:59:40 2009 +0000
@@ -0,0 +1,23 @@
+CPUID emulation for guest
+-------------------------
+
+When HVM guest tries to execute CPUID, or PV guest tries to execute XEN_CPUID,
+the xen hypervior traps and emultes them.
+
+For HVM guest and PV DomU guest, xen's CPUID emulation can be adjusted using
+the guest configation file if necessary (e.g., to supply better support for
+guest live migration). The CPUID syntax in guest configration file is
+described in detail in the examples like /etc/xen/xmexample.hvm,
+/etc/xen/xmexample.hvm-stubdom.
+
+However, a user (or an administrator) must be aware that the CPUID in guest
+configuration file can NOT be configured casually. The default CPUID
+configuration should be safe, but illegal configuration can cause unexpected
+behaviors of guest -- even can crash guest.
+
+For example, we should not expose the MONITOR CPUID feature flag (ECX bit 3;
+CPUID executed EAX = 1) to HVM guest, otherwise, on guest's attempt of
+executing MWAIT, the VMExit handler in Xen would inject #UD (Invalid Opcode
+Exception) into the HVM guest, and guest kernel would panic.
+
+/* We can add more unsafe CPUID configuration here in future. */

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