mm: make opt_bootscrub non-init

LLVM code generation can attempt to load from a variable in the next
condition of an expression under certain circumstances, thus turning
the following condition:

if ( system_state < SYS_STATE_active && opt_bootscrub == BOOTSCRUB_IDLE )

Into:

0xffff82d080223967 <+103>: cmpl   $0x3,0x37b032(%rip) # 0xffff82d08059e9a0 <system_state>
0xffff82d08022396e <+110>: setb   -0x29(%rbp)
0xffff82d080223972 <+114>: cmpl   $0x2,0x228a8b(%rip) # 0xffff82d08044c404 <opt_bootscrub>

Such code will trigger a page fault if system_state >=
SYS_STATE_active because opt_bootscrub will be unmapped.

Fix this by making opt_bootscrub non-init, thus preventing the page
fault. The LLVM bug with the discussion about this issue can be found
at:

https://bugs.llvm.org/show_bug.cgi?id=39707

I haven't been able to find any other instances of such conditional
expression that uses system_state together with an init variable or
function.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

xen/common/page_alloc.c

@@ -166,7 +166,15 @@ enum bootscrub_mode {
     BOOTSCRUB_ON,
     BOOTSCRUB_IDLE,
 };
-static enum bootscrub_mode __initdata opt_bootscrub = BOOTSCRUB_IDLE;
+
+/*
+ * opt_bootscrub should live in the init section, since it's not accessed
+ * afterwards. However at least LLVM assumes there are no side effects of
+ * accessing the variable, and optimizes the condition in init_heap_pages() so
+ * opt_bootscrub is read regardless of the value of system_state:
+ * https://bugs.llvm.org/show_bug.cgi?id=39707
+ */
+static enum bootscrub_mode __read_mostly opt_bootscrub = BOOTSCRUB_IDLE;
 static int __init parse_bootscrub_param(const char *s)
 {
     /* Interpret 'bootscrub' alone in its positive boolean form */