[PATCH] x86/MTRR: constrain AP sync and BSP restore

[Jan Beulich <jbeulich@suse.com>]

mtrr_set_all() has quite a bit of overhead, which is entirely useless
when set_mtrr_state() really does nothing. Furthermore, with
mtrr_state.def_type never initialized from hardware, post_set()'s
unconditional writing of the MSR means would leave us running in UC
mode after the sync.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/cpu/mtrr/main.c
+++ b/xen/arch/x86/cpu/mtrr/main.c
@@ -605,13 +605,15 @@ void mtrr_aps_sync_begin(void)
 
 void mtrr_aps_sync_end(void)
 {
-	set_mtrr(~0U, 0, 0, 0);
+	if (mtrr_if)
+		set_mtrr(~0U, 0, 0, 0);
 	hold_mtrr_updates_on_aps = 0;
 }
 
 void mtrr_bp_restore(void)
 {
-	mtrr_set_all();
+	if (mtrr_if)
+		mtrr_set_all();
 }
 
 static int __init cf_check mtrr_init_finialize(void)