x86/time: always count s_time from Xen boot

Timestamped printks() can call NOW() before init_xen_time().
Set a baseline TSC as soon as we've calibrated the TSC rate,
so that NOW() consistently counts from boot time.

Signed-off-by: Tim Deegan <tim@xen.org>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/boot/head.S

@@ -136,6 +136,12 @@ __start:
         /* Check for availability of long mode. */
         bt      $29,%edx
         jnc     bad_cpu
+
+        /* Stash TSC to calculate a good approximation of time-since-boot */
+        rdtsc
+        mov     %eax,sym_phys(boot_tsc_stamp)
+        mov     %edx,sym_phys(boot_tsc_stamp+4)
+
         /* Initialise L2 boot-map page table entries (16MB). */
         mov     $sym_phys(l2_bootmap),%edx
         mov     $PAGE_HYPERVISOR|_PAGE_PSE,%eax

xen/arch/x86/time.c

@@ -84,6 +84,9 @@ static u16 pit_stamp16;
 static u32 pit_stamp32;
 static bool_t __read_mostly using_pit;
 
+/* Boot timestamp, filled in head.S */
+u64 __initdata boot_tsc_stamp;
+
 /*
  * 32-bit division of integer dividend and integer divisor yielding
  * 32-bit fractional quotient.
@@ -1433,9 +1436,6 @@ int __init init_xen_time(void)
 
     open_softirq(TIME_CALIBRATE_SOFTIRQ, local_time_calibration);
 
-    /* System time (get_s_time()) starts ticking from now. */
-    rdtscll(this_cpu(cpu_time).local_tsc_stamp);
-
     /* NB. get_cmos_time() can take over one second to execute. */
     do_settime(get_cmos_time(), 0, NOW());
 
@@ -1453,9 +1453,11 @@ int __init init_xen_time(void)
 /* Early init function. */
 void __init early_time_init(void)
 {
+    struct cpu_time *t = &this_cpu(cpu_time);
     u64 tmp = init_pit_and_calibrate_tsc();
 
-    set_time_scale(&this_cpu(cpu_time).tsc_scale, tmp);
+    set_time_scale(&t->tsc_scale, tmp);
+    t->local_tsc_stamp = boot_tsc_stamp;
 
     do_div(tmp, 1000);
     cpu_khz = (unsigned long)tmp;