x86/MTRR: constrain AP sync and BSP restore

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>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>

diff --git a/xen/arch/x86/cpu/mtrr/main.c b/xen/arch/x86/cpu/mtrr/main.c
index 0a44ebbcb04ff894da90301fa7593a0d48acc0ab..39b18c6013d4e4be5b743bd65fa2f347af648dd3 100644 (file)
--- 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)