spapr: Explain purpose of ->fwnmi_migration_blocker more clearly

spapr_mce_req_event() makes an effort to prevent migration from
degrading the reporting of FWNMIs.  It adds a migration blocker when
it receives one, and deletes it when it's done handling it.  This is a
best effort.

Commit 2500fb423a "migration: Include migration support for machine
check handling" tried to explain this in a comment.  Rewrite the
comment for clarity, and reposition it to make it clear it applies to
all failure modes, not just "migration already in progress".

Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Aravinda Prasad <arawinda.p@gmail.com>
Cc: Ganesh Goudar <ganeshgr@linux.ibm.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-4-armbru@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>

hw/ppc/spapr_events.c

@@ -911,16 +911,17 @@ void spapr_mce_req_event(PowerPCCPU *cpu, bool recovered)
         }
     }
 
+    /*
+     * Try to block migration while FWNMI is being handled, so the
+     * machine check handler runs where the information passed to it
+     * actually makes sense.  This shouldn't actually block migration,
+     * only delay it slightly, assuming migration is retried.  If the
+     * attempt to block fails, carry on.  Unfortunately, it always
+     * fails when running with -only-migrate.  A proper interface to
+     * delay migration completion for a bit could avoid that.
+     */
     ret = migrate_add_blocker(spapr->fwnmi_migration_blocker, NULL);
     if (ret == -EBUSY) {
-        /*
-         * We don't want to abort so we let the migration to continue.
-         * In a rare case, the machine check handler will run on the target.
-         * Though this is not preferable, it is better than aborting
-         * the migration or killing the VM. It is okay to call
-         * migrate_del_blocker on a blocker that was not added (which the
-         * nmi-interlock handler would do when it's called after this).
-         */
         warn_report("Received a fwnmi while migration was in progress");
     }