kboot: disable WFI/WFIT where it loses state (from AsahiLinux/m1n1)

src/kboot.c

@@ -313,8 +313,37 @@ static int dt_set_chosen(void)
     if (fdt_setprop(dt, node, "asahi,m1n1-stage2-version", m1n1_version, strlen(m1n1_version) + 1))
         bail("FDT: couldn't set asahi,m1n1-stage2-version\n");
 
-    if (dt_set_rng_seed_sep(node))
-        return dt_set_rng_seed_adt(node);
+    if (dt_set_rng_seed_sep(node)) {
+        if (dt_set_rng_seed_adt(node))
+            bail("FDT: couldn't set random seed");
+    }
+
+    if (!cpu_features->apple_sysregs_unlocked) {
+        /*
+         * M4 and later lose architectural state in WFI and WFIT. Add the
+         * bootargs to prevent Linux from crashing in the idle loop and delay
+         * function.
+         */
+        const char to_append[] = " idle=nop arm64.nowfxt";
+        printf("FDT: appending to bootargs:%s\n", to_append);
+
+        /*
+         * fdt_appendprop_string would create another property value:
+         * "a", "b". But we want "ab", so manually create or grow the prop,
+         * then overwrite the \0 of the existing string.
+         */
+        int oldlen, newlen;
+        void *prop_data;
+
+        if (!fdt_get_property(dt, node, "bootargs", &oldlen) || oldlen < 1)
+            oldlen = 1;
+        newlen = oldlen + sizeof(to_append) - 1;
+
+        if (fdt_setprop_placeholder(dt, node, "bootargs", newlen, &prop_data))
+            bail("FDT: failed to append to 'bootargs' property\n");
+
+        strncpy(prop_data + oldlen - 1, to_append, sizeof(to_append));
+    }
 
     return 0;
 }