docs : add doc on cpu model and features

Add documentation on the way libvirt displays the Host CPU
model and capabilities (features). There is an implicit
expectation from users to get the CPU model name matching the
CPU model they are running on, however, this does not happen
most of the time. As a consequence, having a documentation
is useful both for users to align their expectation and for
us to point to a place where the situation is clearly explained.

Signed-off-by: Hector Cao <hector.cao@canonical.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>

docs/formatcaps.rst

@@ -97,6 +97,50 @@ The ``<host/>`` element consists of the following child elements:
    parse this element. In contrast with the former elements, this is repeated
    for each security model the libvirt daemon currently supports.
 
+Host CPU model and features
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+As described previously in this section, libvirt exposes to
+users the list of Host CPU features. Libvirt has a special way to
+expose this list: Instead of providing the full (and thereby often
+very long) set of features, libvirt specifies a CPU model as
+baseline and additional features on top of it (as can be seen in
+`Examples`_ where the baseline is ``Skylake-Client-noTSX-IBRS``).
+
+The ideal case would be that the baseline CPU model definition matches
+exactly the CPU present in the system, and no additional feature is
+needed to express the capabilities of the CPU. For example, if you are
+running on a Server CPU you bought as ``Icelake`` type, the returned
+CPU model name could be ``Icelake-Server``. However, this ideal
+situation rarely happens, for two main reasons:
+
+- Manufacturers do not ship a single type of CPU under a given name,
+  there are various different SKUs with different features under the
+  same name. Yet it is not practical to have a database listing all of
+  those variants.
+
+- Some features might be in the hardware but unavailable for various
+  reasons (BIOS and kernel configuration, disabled for security,
+  ...). One typical example where this situation happens is related to
+  the `TSX mitigation <https://docs.kernel.org/arch/x86/tsx_async_abort.html>`__.
+  As a mitigation to the TAA side channel attack, the Linux kernel disables
+  by default TSX and its 2 features, ``rtm`` and ``hle``. Since many Linux
+  distributions keep this safer default behavior, these 2 features appear
+  as missing.
+
+Because for backward compatibility reasons, host capabilities cannot
+list features that would need to be removed from the baseline model to
+describe the host CPU, libvirt has to often use a rather old CPU
+model, for example, ``Broadwell`` rather than ``Icelake``. Therefore,
+users *should not* expect the reported CPU model name to have any
+implications other than that of a named baseline to build the complete
+available feature set of the Host CPU.
+
+`Domain capabilities <formatdomaincaps.html#cpu-configuration>`_ do
+not have such limitation, and the ``host-model`` CPU definition would
+show the correct CPU model in almost all cases.
+
+
 Guest capabilities
 ~~~~~~~~~~~~~~~~~~