confidential guest support: Introduce cgs "ready" flag

The platform specific details of mechanisms for implementing
confidential guest support may require setup at various points during
initialization.  Thus, it's not really feasible to have a single cgs
initialization hook, but instead each mechanism needs its own
initialization calls in arch or machine specific code.

However, to make it harder to have a bug where a mechanism isn't
properly initialized under some circumstances, we want to have a
common place, late in boot, where we verify that cgs has been
initialized if it was requested.

This patch introduces a ready flag to the ConfidentialGuestSupport
base type to accomplish this, which we verify in
qemu_machine_creation_done().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>

include/exec/confidential-guest-support.h

@@ -27,6 +27,30 @@ OBJECT_DECLARE_SIMPLE_TYPE(ConfidentialGuestSupport, CONFIDENTIAL_GUEST_SUPPORT)
 
 struct ConfidentialGuestSupport {
     Object parent;
+
+    /*
+     * ready: flag set by CGS initialization code once it's ready to
+     *        start executing instructions in a potentially-secure
+     *        guest
+     *
+     * The definition here is a bit fuzzy, because this is essentially
+     * part of a self-sanity-check, rather than a strict mechanism.
+     *
+     * It's not feasible to have a single point in the common machine
+     * init path to configure confidential guest support, because
+     * different mechanisms have different interdependencies requiring
+     * initialization in different places, often in arch or machine
+     * type specific code.  It's also usually not possible to check
+     * for invalid configurations until that initialization code.
+     * That means it would be very easy to have a bug allowing CGS
+     * init to be bypassed entirely in certain configurations.
+     *
+     * Silently ignoring a requested security feature would be bad, so
+     * to avoid that we check late in init that this 'ready' flag is
+     * set if CGS was requested.  If the CGS init hasn't happened, and
+     * so 'ready' is not set, we'll abort.
+     */
+    bool ready;
 };
 
 typedef struct ConfidentialGuestSupportClass {