docs/sphinx: Start an FAQ, and add Kconfig/CET details

This is long overdue, and we need to start somewhere.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

docs/faq.rst

@@ -0,0 +1,71 @@
+.. SPDX-License-Identifier: CC-BY-4.0
+
+Frequently Asked Questions
+==========================
+
+How do I...
+-----------
+
+... check whether a Kconfig option is active?
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+  Kconfig is a build time configuration system, combining inherent knowledge,
+  the capabilities of the toolchain, and explicit user choice to form a
+  configuration of a build of Xen.
+
+  A file, by default ``.config``, is produced by the build identifying the
+  configuration used.  Kconfig symbols all start with ``CONFIG_``, and come in
+  a variety of types including strings, integers and booleans.  Booleans are
+  the most common, and when active are expressed with ``...=y``.  e.g.::
+
+    xen.git/xen$ grep CONFIG_FOO .config
+    CONFIG_FOO_BOOLEAN=y
+    CONFIG_FOO_STRING="lorem ipsum"
+    CONFIG_FOO_INTEGER=42
+
+  Symbols which are either absent, or expressed as ``... is not set`` are
+  disabled.  e.g.::
+
+    xen.git/xen$ grep CONFIG_BAR .config
+    # CONFIG_BAR is not set
+
+  Builds of Xen configured with ``CONFIG_HYPFS_CONFIG=y`` embed their own
+  ``.config`` at build time, and can provide it to the :term:`control domain`
+  upon requested.  e.g.::
+
+    [root@host ~]# xenhypfs cat /buildinfo/config | grep -e FOO -e BAR
+    CONFIG_FOO=y
+    # CONFIG_BAR is not set
+
+
+... tell if CET is active?
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+  Control-flow Enforcement Technology support was added to Xen 4.14.  It is
+  build time conditional, dependent on both having a new-enough toolchain and
+  an explicit Kconfig option, and also requires capable hardware.  See
+  :term:`CET`.
+
+  For CET-SS, Shadow Stacks, the minimum toolchain requirements are ``binutils
+  >= 2.29`` or ``LLVM >= 6``.  No specific compiler support is required.
+  Check for ``CONFIG_XEN_SHSTK`` being active.
+
+  For CET-IBT, Indirect Branch Tracking, the minimum toolchain requirements
+  are ``GCC >= 9`` and ``binutils >= 2.29``.  Xen relies on a compiler feature
+  which is specific to GCC at the time of writing.  Check for
+  ``CONFIG_XEN_IBT`` being active.
+
+  If a capable Xen is booted on capable hardware, and CET is not disabled by
+  command line option or errata, Xen will print some details early on boot
+  about which CET facilities have been turned on::
+
+    ...
+    (XEN) CPU Vendor: Intel, Family 6 (0x6), Model 143 (0x8f), Stepping 8 (raw 000806f8)
+    (XEN) Enabling Supervisor Shadow Stacks
+    (XEN) Enabling Indirect Branch Tracking
+    (XEN)   - IBT disabled in UEFI Runtime Services
+    (XEN) EFI RAM map:
+    ...
+
+  This can be obtained from the control domain with ``xl dmesg``, but remember
+  to confirm that the console ring hasn't wrapped.