[PATCH v2 2/5] x86/time: Always count s_time from Xen boot

[Andrew Cooper <andrew.cooper3@citrix.com>]

From: Tim Deegan <tim@xen.org>

In the early-boot clock, before the calibration routines kick in,
count time from Xen boot rather than from when the BSP's TSC was 0.

Signed-off-by: Tim Deegan <tim@xen.org>

Stash the timestamp in head.S as a good approximation of when we actually
started, rather than measuing "time since we ran early_time_init()"

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

CC: Keir Fraser <keir@xen.org>
CC: Jan Beulich <JBeulich@suse.com>
---
 xen/arch/x86/boot/head.S |   10 ++++++++++
 xen/arch/x86/time.c      |    4 ++++
 2 files changed, 14 insertions(+)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index b12eefb..d62eaae 100644
--- a/xen/arch/x86/boot/head.S
+++ b/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
@@ -203,6 +209,10 @@ GLOBAL(trampoline_end)
 __high_start:
 #include "x86_64.S"
 
+        .section .init.data, "a", @progbits
+GLOBAL(boot_tsc_stamp)
+        .quad   0
+
         .section .data.page_aligned, "aw", @progbits
         .p2align PAGE_SHIFT
 /*
diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 883c135..3da4def 100644
--- a/xen/arch/x86/time.c
+++ b/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 (initdata) */
+extern u64 boot_tsc_stamp;
+
 /*
  * 32-bit division of integer dividend and integer divisor yielding
  * 32-bit fractional quotient.
@@ -1456,6 +1459,7 @@ void __init early_time_init(void)
     u64 tmp = init_pit_and_calibrate_tsc();
 
     set_time_scale(&this_cpu(cpu_time).tsc_scale, tmp);
+    this_cpu(cpu_time).local_tsc_stamp = boot_tsc_stamp;
 
     do_div(tmp, 1000);
     cpu_khz = (unsigned long)tmp;
-- 
1.7.10.4