[Xen-devel] [PATCH v3] x86/time: Always count s_time from Xen boot

[Tim Deegan <tim@xxxxxxx>]

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@xxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
v2: use TSC from __start() rather than from early_time_init().
v3: remove later reset of TSC baseline from init_xen_time().

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 000191b..a5e7eca 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.
@@ -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());
 
@@ -1456,6 +1456,7 @@ 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;

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