xen/arm: gic: Add GIC SGI boot/self tests in Xen

Boot self-tests (also referred to as boot-time tests or power-on
self-tests) are intended to check that Xen has configured the hardware
correctly before bringing up any domains.

Introduce tests to confirm that, using a dedicated SGI (GIC_SGI_TEST):
1. A cpu can send the SGI to itself
2. A cpu can send the SGI to another specific CPU (CPU0)
3. A cpu can send the SGI to all the other CPUs

Each CPU counts the test SGIs it takes. A sender samples those counters
before sending and then waits for the count of every CPU it targeted to
change, which is how it tells that the SGI was really delivered. The
counters are never reset, so comparing against a sample rather than an
absolute value keeps concurrent senders from disturbing each other. The
snapshot is allocated rather than kept on the stack, as NR_CPUS can be
large enough for such an array to overflow it.

A test reports a failure by panic(), so Xen never continues on a
platform where SGI delivery is broken. When the tests pass, Xen carries
on booting normally.

Also introduce a config CONFIG_BOOT_SELFTEST which enables these
tests. It depends on DEBUG and is off unless explicitly enabled.

Also introduce a boolean command line parameter "gic-test", so that a
build with CONFIG_BOOT_SELFTEST enabled can be shipped but the tests
selected at boot. It is documented in
docs/misc/xen-command-line.pandoc.

In order to keep all the boot self-tests together in the binary, a
separate section "initcallboottest" is introduced. Tests are registered
using __initcallboottest() and run once on every CPU by
do_init_boottests(), bracketed by begin/end messages. They run before
any domain is created on the primary core, and before the idle loop is
entered on a secondary core.

Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Julien Grall <julien@xen.org>

docs/misc/xen-command-line.pandoc

@@ -1282,6 +1282,16 @@ available on Intel Panther Lake and Diamond Rapids CPUs, and AMD Zen6 CPUs.
 FRED is fully supported on AMD hardware.  On Intel hardware it is still tech
 preview, and in particular not security supported.
 
+### gic-test (arm)
+> `= <boolean>`
+
+> Default: `false`
+
+Only available when `CONFIG_BOOT_SELFTEST` is enabled.
+
+Run the GIC SGI boot self-tests while each CPU is brought up.  Xen panics if
+an SGI is not delivered; otherwise it carries on booting normally.
+
 ### gnttab
 > `= List of [ max-ver:<integer>, transitive=<bool>, transfer=<bool> ]`