Signed-off-by: Jiri Denemark <jdenemar@xxxxxxxxxx>
---
Notes:
Version 3:
- reworded and updated documentation
Version 2:
- no change
docs/formatdomain.html.in | 37 ++++++++++++++++++++++++-------------
1 file changed, 24 insertions(+), 13 deletions(-)
diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index 02ce7924c..ee631186c 100644
--- a/docs/formatdomain.html.in
+++ b/docs/formatdomain.html.in
@@ -1272,19 +1272,21 @@
model even if the destination host contains more capable CPUs for
the running instance of the guest; but shutting down and restarting
the guest may present different hardware to the guest according to
- the capabilities of the new host. <strong>Beware</strong>, due to the
- way libvirt detects host CPU and due to the fact libvirt does not
- talk to QEMU/KVM when creating the CPU model, CPU configuration
- created using <code>host-model</code> may not work as expected. The
- guest CPU may differ from the configuration and it may also confuse
- guest OS by using a combination of CPU features and other parameters
- (such as CPUID level) that don't work. Until these issues are fixed,
- it's a good idea to avoid using <code>host-model</code> and use
- <code>custom</code> mode with just the CPU model from host
- capabilities XML.
- <span class="since">Since 1.2.11</span> PowerISA allows
- processors to run VMs in binary compatibility mode supporting an
- older version of ISA. Libvirt on PowerPC architecture uses the
+ the capabilities of the new host. Prior to libvirt 3.1.0 and QEMU
+ 2.9.0 detection of the host CPU model via QEMU is not supported.
+ Thus the CPU configuration created using <code>host-model</code>
+ may not work as expected.
+ <span class="since">Since 3.1.0 and QEMU 2.9.0</span> this mode
+ works the way it was designed and it is indicated by the
+ <code>fallback</code> attribute set to <code>forbid</code> in the
+ host-model CPU definition advertised in
+ <a href="formatdomaincaps.html#elementsCPU">domain capabilities XML</a>.
+ When <code>fallback</code> attribute is set to <code>allow</code>
+ in the domain capabilities XML, it is recommended to use
+ <code>custom</code> mode with just the CPU model from the host
+ capabilities XML. <span class="since">Since 1.2.11</span> PowerISA
+ allows processors to run VMs in binary compatibility mode supporting
+ an older version of ISA. Libvirt on PowerPC architecture uses the
<code>host-model</code> to signify a guest mode CPU running in
binary compatibility mode. Example:
When a user needs a power7 VM to run in compatibility mode
@@ -1307,6 +1309,15 @@
a migration is attempted then the guest may hang or crash upon
resuming execution on the destination host.</dd>
</dl>
+
+ Both <code>host-model</code> and <code>host-passthrough</code> modes
+ make sense when a domain can run directly on the host CPUs (for
+ example, domains with type <code>kvm</code>). The actual host CPU is
+ irrelevant for domains with emulated virtual CPUs (such as domains with
+ type <code>qemu</code>). However, for backward compatibility
+ <code>host-model</code> may be implemented even for domains running on
+ emulated CPUs in which case the best CPU the hypervisor is able to
+ emulate may be used rather then trying to mimic the host CPU model.
</dd>
<dt><code>model</code></dt>
--
2.11.1
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