[libvirt] [PATCH 08/33] docs: Update description of the host-model CPU mode
Signed-off-by: Jiri Denemark <jdenemar(a)redhat.com>
---
docs/formatdomain.html.in | 28 ++++++++++++++++++----------
1 file changed, 18 insertions(+), 10 deletions(-)
diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index 294d3c467..9e7f7804e 100644
--- a/docs/formatdomain.html.in
+++ b/docs/formatdomain.html.in
@@ -1272,16 +1272,15 @@
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.
+ the capabilities of the new host. Libvirt older than 3.1.0 or QEMU
+ older than 2.9.0 do not support detection of the host CPU model via
+ QEMU and thus the CPU configuration created using
+ <code>host-model</code> may not work as expected.
+ <span class="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>.
<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
@@ -1307,6 +1306,15 @@
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 without
+ (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