[Xen-devel] [PATCH v2 3/6] x86/time: implement tsc as clocksource

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

Introduce support for using TSC as platform time which is the highest
resolution time and most performant to get (~20 nsecs).  Though there
are also several problems associated with its usage, and there isn't a
complete (and architecturally defined) guarantee that all machines
will provide reliable and monotonic TSC across all CPUs, on different
sockets and different P/C states.  I believe Intel to be the only that
can guarantee that. For this reason it's only set when adminstrator
changes "clocksource" boot option to "tsc". Initializing TSC
clocksource requires all CPUs to have the tsc reliability checks
performed. init_xen_time is called before all CPUs are up, and so we
start with HPET at boot time, and switch later to TSC. The switch then
happens on verify_tsc_reliability initcall that is invoked when all
CPUs are up. When attempting to initializing TSC we also check for
time warps and appropriate CPU features i.e.  TSC_RELIABLE,
CONSTANT_TSC and NONSTOP_TSC. And in case none of these conditions are
met, we keep the clocksource that was previously initialized on
init_xen_time.

It is also worth noting that with clocksource=tsc there isn't any
need to synchronise with another clocksource, and I could verify that
great portion the time skew was eliminated and seeing much less time
warps happening. With HPET I used to observe ~500 warps in the period
of 1h of around 27 us, and with TSC down to 50 warps in the same
period having each warp < 100 ns. The warps still exist though but
are only related to cross CPU calibration (being how much it takes to
rendezvous with master), in which a later patch in this series aims to
solve.

Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
---
Cc: Keir Fraser <keir@xxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Changes since v1:
 - s/printk/printk(XENLOG_INFO
 - Remove extra space on inner brackets
 - Add missing space around brackets
 - Defer clocksource TSC initialization when all CPUs are up.

Changes since RFC:
 - Spelling fixes in the commit message.
 - Remove unused clocksource_is_tsc variable and introduce it instead
 on the patch that uses it.
 - Move plt_tsc from second to last in the available clocksources.
---
 xen/arch/x86/time.c | 97 +++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 95 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index ed4ed24..2602dda 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -432,6 +432,63 @@ uint64_t ns_to_acpi_pm_tick(uint64_t ns)
 }
 
 /************************************************************
+ * PLATFORM TIMER 4: TSC
+ */
+static u64 tsc_freq;
+static unsigned long tsc_max_warp;
+static void tsc_check_reliability(void);
+
+static int __init init_tsctimer(struct platform_timesource *pts)
+{
+    bool_t tsc_reliable = 0;
+
+    tsc_check_reliability();
+
+    if ( tsc_max_warp > 0 )
+    {
+        tsc_reliable = 0;
+        printk(XENLOG_INFO "TSC: didn't passed warp test\n");
+    }
+    else if ( boot_cpu_has(X86_FEATURE_TSC_RELIABLE) ||
+              (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
+               boot_cpu_has(X86_FEATURE_NONSTOP_TSC)) )
+    {
+        tsc_reliable = 1;
+    }
+    else if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
+    {
+        tsc_reliable = (max_cstate <= 2);
+
+        if ( tsc_reliable )
+            printk(XENLOG_INFO "TSC: no deep Cstates, deemed reliable\n");
+        else
+            printk(XENLOG_INFO "TSC: deep Cstates possible, so not 
reliable\n");
+    }
+
+    pts->frequency = tsc_freq;
+    return tsc_reliable;
+}
+
+static u64 read_tsc(void)
+{
+    return rdtsc();
+}
+
+static void resume_tsctimer(struct platform_timesource *pts)
+{
+}
+
+static struct platform_timesource __initdata plt_tsc =
+{
+    .id = "tsc",
+    .name = "TSC",
+    .read_counter = read_tsc,
+    .counter_bits = 64,
+    .init = init_tsctimer,
+    .resume = resume_tsctimer,
+};
+
+/************************************************************
  * GENERIC PLATFORM TIMER INFRASTRUCTURE
  */
 
@@ -533,6 +590,21 @@ static void resume_platform_timer(void)
     plt_stamp = plt_src.read_counter();
 }
 
+static void __init reset_platform_timer(void)
+{
+    /* Deactivate any timers running */
+    kill_timer(&plt_overflow_timer);
+    kill_timer(&calibration_timer);
+
+    /* Reset counters and stamps */
+    spin_lock_irq(&platform_timer_lock);
+    plt_stamp = 0;
+    plt_stamp64 = 0;
+    platform_timer_stamp = 0;
+    stime_platform_stamp = 0;
+    spin_unlock_irq(&platform_timer_lock);
+}
+
 static int __init try_platform_timer(struct platform_timesource *pts)
 {
     int rc = -1;
@@ -541,6 +613,10 @@ static int __init try_platform_timer(struct 
platform_timesource *pts)
     if ( rc <= 0 )
         return rc;
 
+    /* We have a platform timesource already so reset it */
+    if ( plt_src.counter_bits != 0 )
+        reset_platform_timer();
+
     plt_mask = (u64)~0ull >> (64 - pts->counter_bits);
 
     set_time_scale(&plt_scale, pts->frequency);
@@ -566,7 +642,9 @@ static void __init init_platform_timer(void)
     struct platform_timesource *pts = NULL;
     int i, rc = -1;
 
-    if ( opt_clocksource[0] != '\0' )
+    /* clocksource=tsc is initialized later when all CPUS are up */
+    if ( (opt_clocksource[0] != '\0') &&
+         (strcmp(opt_clocksource, "tsc") != 0) )
     {
         for ( i = 0; i < ARRAY_SIZE(plt_timers); i++ )
         {
@@ -1192,7 +1270,7 @@ static void check_tsc_warp(unsigned long tsc_khz, 
unsigned long *max_warp)
     }
 }
 
-static unsigned long tsc_max_warp, tsc_check_count;
+static unsigned long tsc_check_count;
 static cpumask_t tsc_check_cpumask;
 
 static void tsc_check_slave(void *unused)
@@ -1437,6 +1515,20 @@ static int __init verify_tsc_reliability(void)
         }
     }
 
+    if ( !strcmp(opt_clocksource, "tsc") )
+    {
+        if ( try_platform_timer(&plt_tsc) > 0 )
+        {
+            printk(XENLOG_INFO "Switched to Platform timer %s TSC\n",
+                   freq_string(plt_src.frequency));
+
+            init_percpu_time();
+
+            init_timer(&calibration_timer, time_calibration, NULL, 0);
+            set_timer(&calibration_timer, NOW() + EPOCH);
+        }
+    }
+
     return 0;
 }
 __initcall(verify_tsc_reliability);
@@ -1476,6 +1568,7 @@ void __init early_time_init(void)
     struct cpu_time *t = &this_cpu(cpu_time);
     u64 tmp = init_pit_and_calibrate_tsc();
 
+    tsc_freq = tmp;
     set_time_scale(&t->tsc_scale, tmp);
     t->local_tsc_stamp = boot_tsc_stamp;
 
-- 
2.1.4


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