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A Timecounter for Xen
I have here a patch that adds timecounter support for xen; it's been
gathering dust in a local tree of mine for a few months, but before
life happened and I got sidetracked from it, it stood up to a fair
amount of testing, and it still works. The changes, while mostly
localized, are not trivial, so I'm posting them here.
As always, it returns Xen "system time" -- monotonic nanoseconds since
boot, measured with the x86 cycle counter and scaling information
provided and maintained by the hypervisor.
The main subtlety is that, because a domain might not be scheduled to
run until some time after the timer interrupt-equivalent should occur,
if the MI timecounter tick routine (called under hardclock(9)) got the
actual times, it could miscompute the conversion to wall clock time.
Thus, the timer event handler arranges to have those timecount values
offset by the difference, as if the hardclock tick had occurred when
it was supposed it.
Also, because such adjustments can't be done for arbitrary other
timecounters (e.g., ACPI timers in Xen3 dom0), the xen_system_time
counter assigns itself a very high quality in order to prevent their
use by default.
It's not SMP-ready at the moment (and hadn't been before this change),
but that should be fairly easy to add; it'd be mostly a simple matter
of moving static globals into cpu_info.
--
(let ((C call-with-current-continuation)) (apply (lambda (x y) (x y)) (map
((lambda (r) ((C C) (lambda (s) (r (lambda l (apply (s s) l)))))) (lambda
(f) (lambda (l) (if (null? l) C (lambda (k) (display (car l)) ((f (cdr l))
(C k))))))) '((#\J #\d #\D #\v #\s) (#\e #\space #\a #\i #\newline)))))
Index: arch/xen/conf/files.xen
===================================================================
RCS file: /lfs/nb/repo/src/sys/arch/xen/conf/files.xen,v
retrieving revision 1.59
diff -u -p -r1.59 files.xen
--- arch/xen/conf/files.xen 16 Apr 2007 19:12:19 -0000 1.59
+++ arch/xen/conf/files.xen 9 May 2007 04:02:48 -0000
@@ -48,7 +48,6 @@ file arch/xen/i386/machdep.c
file arch/xen/i386/identcpu.c
file arch/i386/i386/math_emulate.c math_emulate
file arch/i386/i386/mem.c
-file kern/kern_microtime.c i586_cpu | i686_cpu
file arch/i386/i386/mtrr_k6.c mtrr
file netns/ns_cksum.c ns
file arch/xen/i386/pmap.c
Index: arch/xen/i386/machdep.c
===================================================================
RCS file: /lfs/nb/repo/src/sys/arch/xen/i386/machdep.c,v
retrieving revision 1.37
diff -u -p -r1.37 machdep.c
--- arch/xen/i386/machdep.c 5 Mar 2007 03:31:29 -0000 1.37
+++ arch/xen/i386/machdep.c 9 May 2007 04:02:48 -0000
@@ -268,7 +268,6 @@ void (*microtime_func)(struct timeval *)
void (*initclock_func)(void) = i8254_initclocks;
#else
void (*delay_func)(int) = xen_delay;
-void (*microtime_func)(struct timeval *) = xen_microtime;
void (*initclock_func)(void) = xen_initclocks;
#endif
Index: arch/xen/include/cpu.h
===================================================================
RCS file: /lfs/nb/repo/src/sys/arch/xen/include/cpu.h,v
retrieving revision 1.18
diff -u -p -r1.18 cpu.h
--- arch/xen/include/cpu.h 12 Mar 2007 16:43:11 -0000 1.18
+++ arch/xen/include/cpu.h 9 May 2007 04:02:48 -0000
@@ -58,7 +58,6 @@
#include <sys/device.h>
#include <sys/simplelock.h> /* will also get LOCKDEBUG */
#include <sys/cpu_data.h>
-#include <sys/cc_microtime.h>
#include <lib/libkern/libkern.h> /* offsetof */
@@ -85,7 +84,6 @@ struct cpu_info {
cpuid_t ci_cpuid; /* our CPU ID */
u_int ci_apicid; /* our APIC ID */
struct cpu_data ci_data; /* MI per-cpu data */
- struct cc_microtime_state ci_cc;/* cc_microtime state */
/*
* Private members.
@@ -283,12 +281,9 @@ extern void cpu_signotify(struct lwp *);
* We need a machine-independent name for this.
*/
extern void (*delay_func)(int);
-struct timeval;
-extern void (*microtime_func)(struct timeval *);
#define DELAY(x) (*delay_func)(x)
#define delay(x) (*delay_func)(x)
-#define microtime(tv) (*microtime_func)(tv)
/*
* pull in #defines for kinds of processors
@@ -373,7 +368,6 @@ void i8254_initclocks(void);
#else
void startrtclock(void);
void xen_delay(int);
-void xen_microtime(struct timeval *);
void xen_initclocks(void);
#endif
Index: arch/xen/include/types.h
===================================================================
RCS file: /lfs/nb/repo/src/sys/arch/xen/include/types.h,v
retrieving revision 1.4
diff -u -p -r1.4 types.h
--- arch/xen/include/types.h 9 Feb 2007 21:55:16 -0000 1.4
+++ arch/xen/include/types.h 9 May 2007 04:02:48 -0000
@@ -79,4 +79,6 @@ typedef volatile unsigned char __cpu_si
#define __HAVE_RAS
#endif
+#define __HAVE_TIMECOUNTER
+
#endif /* _MACHTYPES_H_ */
Index: arch/xen/xen/clock.c
===================================================================
RCS file: /lfs/nb/repo/src/sys/arch/xen/xen/clock.c,v
retrieving revision 1.34
diff -u -p -r1.34 clock.c
--- arch/xen/xen/clock.c 11 Nov 2006 23:33:12 -0000 1.34
+++ arch/xen/xen/clock.c 9 May 2007 04:02:49 -0000
@@ -39,6 +39,8 @@ __KERNEL_RCSID(0, "$NetBSD: clock.c,v 1.
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/time.h>
+#include <sys/timetc.h>
+#include <sys/timevar.h>
#include <sys/kernel.h>
#include <sys/device.h>
#include <sys/sysctl.h>
@@ -54,17 +56,40 @@ __KERNEL_RCSID(0, "$NetBSD: clock.c,v 1.
static int xen_timer_handler(void *, struct intrframe *);
+/* A timecounter: Xen system_time extrapolated with a TSC. */
+u_int xen_get_timecount(struct timecounter*);
+static struct timecounter xen_timecounter = {
+ .tc_get_timecount = xen_get_timecount,
+ .tc_poll_pps = NULL,
+ .tc_counter_mask = ~0U,
+ .tc_frequency = 1000000000ULL,
+ .tc_name = "xen_system_time",
+ .tc_quality = 10000 /*
+ * This needs to take precedence over any hardware
+ * timecounters (e.g., ACPI in Xen3 dom0), because
+ * they can't correct for Xen scheduling latency.
+ */
+};
+
/* These are periodically updated in shared_info, and then copied here. */
static volatile uint64_t shadow_tsc_stamp;
static volatile uint64_t shadow_system_time;
static volatile unsigned long shadow_time_version; /* XXXSMP */
static volatile uint32_t shadow_freq_mul;
static volatile int8_t shadow_freq_shift;
-static volatile struct timeval shadow_tv;
+static volatile struct timespec shadow_ts;
static int timeset;
-static uint64_t processed_system_time;
+/* The time when the last hardclock(9) call should have taken place. */
+static volatile uint64_t processed_system_time;
+
+/*
+ * The clock (as returned by xen_get_timecount) may need to be held
+ * back to maintain the illusion that hardclock(9) was called when it
+ * was supposed to be, not when Xen got around to scheduling us.
+ */
+static volatile uint64_t xen_clock_bias = 0;
#ifdef DOM0OPS
/* If we're dom0, send our time to Xen every minute or so. */
@@ -76,7 +101,7 @@ static struct callout xen_timepush_co =
/*
* Reads a consistent set of time-base values from Xen, into a shadow data
- * area. Must be called at splclock.
+ * area. Must be called at splhigh (per timecounter requirements).
*/
static void
get_time_values_from_xen(void)
@@ -98,22 +123,22 @@ get_time_values_from_xen(void)
do {
tversion = HYPERVISOR_shared_info->wc_version;
x86_lfence();
- shadow_tv.tv_sec = HYPERVISOR_shared_info->wc_sec;
- shadow_tv.tv_usec = HYPERVISOR_shared_info->wc_nsec;
+ shadow_ts.tv_sec = HYPERVISOR_shared_info->wc_sec;
+ shadow_ts.tv_nsec = HYPERVISOR_shared_info->wc_nsec;
x86_lfence();
} while ((HYPERVISOR_shared_info->wc_version & 1) ||
(tversion != HYPERVISOR_shared_info->wc_version));
- shadow_tv.tv_usec = shadow_tv.tv_usec / 1000;
#else /* XEN3 */
do {
shadow_time_version = HYPERVISOR_shared_info->time_version2;
x86_lfence();
- shadow_tv.tv_sec = HYPERVISOR_shared_info->wc_sec;
- shadow_tv.tv_usec = HYPERVISOR_shared_info->wc_usec;
+ shadow_ts.tv_sec = HYPERVISOR_shared_info->wc_sec;
+ shadow_ts.tv_nsec = HYPERVISOR_shared_info->wc_usec;
shadow_tsc_stamp = HYPERVISOR_shared_info->tsc_timestamp;
shadow_system_time = HYPERVISOR_shared_info->system_time;
x86_lfence();
} while (shadow_time_version != HYPERVISOR_shared_info->time_version1);
+ shadow_ts.tv_nsec *= 1000;
#endif
}
@@ -163,6 +188,10 @@ scale_delta(uint64_t delta, uint32_t mul
}
#endif
+/*
+ * Use cycle counter to determine ns elapsed since last Xen time update.
+ * Must be called at splhigh (per timecounter requirements).
+ */
static uint64_t
get_tsc_offset_ns(void)
{
@@ -179,37 +208,79 @@ get_tsc_offset_ns(void)
shadow_freq_shift);
#endif
#ifdef XEN_CLOCK_DEBUG
- if (offset > 10000000000ULL)
- printf("get_tsc_offset_ns: tsc_delta=%llu offset=%llu\n",
- tsc_delta, offset);
+ if (tsc_delta > 100000000000ULL || offset > 10000000000ULL)
+ printf("get_tsc_offset_ns: tsc_delta=%llu offset=%llu"
+ " pst=%llu sst=%llu\n", tsc_delta, offset,
+ processed_system_time, shadow_system_time);
#endif
+
return offset;
}
+/*
+ * Returns the current system_time, taking care that the timestamp
+ * used is valid for the TSC measurement in question. Xen2 doesn't
+ * ensure that this won't step backwards, so we enforce monotonicity
+ * on our own in that case. Must be called at splhigh.
+ */
static uint64_t
get_system_time(void)
{
- uint64_t stime;
-
+#ifndef XEN3
+ static volatile uint64_t oldstime = 0;
+#endif
+ uint64_t offset, stime;
+
for (;;) {
- stime = shadow_system_time + get_tsc_offset_ns();
+ offset = get_tsc_offset_ns();
+ stime = shadow_system_time + offset;
/* if the timestamp went stale before we used it, refresh */
- if (time_values_up_to_date())
+ if (time_values_up_to_date()) {
+ /*
+ * Work around an intermittent Xen2 bug where, for
+ * a period of 1<<32 ns, currently running domains
+ * don't get their timer events as usual (and also
+ * aren't preempted in favor of other runnable
+ * domains). Setting the timer into the past in
+ * this way causes it to fire immediately.
+ */
+#ifndef XEN3
+ if (offset > 4*10000000ULL) {
+#ifdef XEN_CLOCK_DEBUG
+ printf("get_system_time: overlarge offset %llu"
+ " (pst=%llu sst=%llu); poking timer...\n",
+ offset, processed_system_time,
+ shadow_system_time);
+#endif
+ HYPERVISOR_set_timer_op(shadow_system_time);
+ }
+#endif
break;
+ }
get_time_values_from_xen();
}
+
+#ifndef XEN3
+ if (stime < oldstime) {
+#ifdef XEN_CLOCK_DEBUG
+ printf("xen_get_timecount: system_time backstep: %"
+ PRIu64" -> %"PRIu64" (%"PRIu64" ns)\n",
+ oldstime, stime, oldstime-stime);
+#endif
+ stime = oldstime;
+ }
+ oldstime = stime;
+#endif
+
return stime;
}
void
inittodr(time_t base)
{
+ struct timespec sts;
int s;
- struct cpu_info *ci = curcpu();
-#if defined(XEN3)
- uint64_t t;
-#endif /* defined(XEN3) */
/*
* if the file system time is more than a year older than the
@@ -220,28 +291,16 @@ inittodr(time_t base)
base = CONFIG_TIME;
}
- s = splclock();
+ s = splhigh();
get_time_values_from_xen();
+ sts = shadow_ts;
splx(s);
-#if defined(XEN3)
- t = (shadow_tv.tv_sec + rtc_offset * 60) * UINT64_C(1000000) +
- shadow_tv.tv_usec + processed_system_time / 1000;
- time.tv_usec = t % UINT64_C(1000000);
- time.tv_sec = t / UINT64_C(1000000);
-#else /* defined(XEN3) */
- time.tv_usec = shadow_tv.tv_usec;
- time.tv_sec = shadow_tv.tv_sec + rtc_offset * 60;
-#endif /* defined(XEN3) */
-#ifdef XEN_CLOCK_DEBUG
- printf("readclock: %ld (%ld)\n", time.tv_sec, base);
-#endif
- /* reset microset, so that the next call to microset() will init */
- ci->ci_cc.cc_denom = 0;
-
- if (base != 0 && base < time.tv_sec - 5*SECYR)
+ tc_setclock(&sts); /* XXX what about rtc_offset? */
+
+ if (base != 0 && base < time_second - 5*SECYR)
printf("WARNING: file system time much less than clock time\n");
- else if (base > time.tv_sec + 5*SECYR) {
+ else if (base > time_second + 5*SECYR) {
printf("WARNING: clock time much less than file system time\n");
printf("WARNING: using file system time\n");
goto fstime;
@@ -252,12 +311,13 @@ inittodr(time_t base)
fstime:
timeset = 1;
- time.tv_sec = base;
+ sts.tv_sec = base;
+ tc_setclock(&sts);
printf("WARNING: CHECK AND RESET THE DATE!\n");
}
-static void
-resettodr_i(void)
+void
+resettodr(void)
{
#ifdef DOM0OPS
dom0_op_t op;
@@ -274,8 +334,8 @@ resettodr_i(void)
if (!timeset)
return;
-#ifdef DEBUG_CLOCK
- {
+#ifdef XXX_DEBUG_CLOCK
+ { /* XXX annoying debug printf not yet timecounterized */
char pm;
if (timercmp(&time, &shadow_tv, >)) {
@@ -291,38 +351,25 @@ resettodr_i(void)
#endif
#ifdef DOM0OPS
if (xen_start_info.flags & SIF_PRIVILEGED) {
- s = splclock();
+ struct timespec now;
op.cmd = DOM0_SETTIME;
- op.u.settime.secs = time.tv_sec - rtc_offset * 60;
+ nanotime(&now);
+ /* XXX is rtc_offset handled correctly everywhere? */
+ op.u.settime.secs = now.tv_sec - rtc_offset * 60;
#ifdef XEN3
- op.u.settime.nsecs = time.tv_usec * 1000;
+ op.u.settime.nsecs = now.tv_nsec;
#else
- op.u.settime.usecs = time.tv_usec;
+ op.u.settime.usecs = now.tv_nsec / 1000;
#endif
- op.u.settime.system_time = processed_system_time;
- HYPERVISOR_dom0_op(&op);
-
+ s = splhigh();
+ op.u.settime.system_time = get_system_time();
splx(s);
+ HYPERVISOR_dom0_op(&op);
}
#endif
}
-/*
- * When the clock is administratively set, in addition to resetting
- * Xen's clock if possible, we should also allow xen_microtime to
- * step backwards without complaint.
- */
-void
-resettodr()
-{
- CPU_INFO_ITERATOR cii;
- struct cpu_info *ci;
-
- resettodr_i();
- for (CPU_INFO_FOREACH(cii, ci))
- timerclear(&ci->ci_cc.cc_time);
-}
void
startrtclock()
@@ -360,100 +407,19 @@ xen_delay(int n)
return;
} else {
uint64_t when;
-
+ int s;
/* for large delays, shadow_system_time is OK */
+
+ s = splhigh();
get_time_values_from_xen();
when = shadow_system_time + n * 1000;
- while (shadow_system_time < when)
+ while (shadow_system_time < when) {
+ splx(s);
+ s = splhigh();
get_time_values_from_xen();
- }
-}
-
-/*
- * A MD microtime for xen.
- *
- * This abuses/reuses the cc_microtime fields already in cpuinfo:
- * cc_ms_delta = usec added to time(9) on last call to hardclock;
- * this is used to scale the actual elapsed time
- * cc_cc = reference value of cpu_counter()
- * cc_denom = nsec between last hardclock(9) and time of cc_cc setting
- * (provided by Xen)
- *
- * We are taking Xen's word for the CPU frequency rather than trying to
- * time it ourselves like cc_microtime does, since Xen could reschedule
- * our virtual CPU(s) onto any physical CPU and only tell us afterwards
- * with a clock interrupt -- and that could invalidate all stored
- * cpu_counter values.
- */
-void
-xen_microtime(struct timeval *tv)
-{
- int s = splclock();
- struct cpu_info *ci = curcpu();
- int64_t cycles;
-
- *tv = time;
- /* Extrapolate from hardclock()'s last step. */
- cycles = cpu_counter() - ci->ci_cc.cc_cc;
-#ifdef XEN_CLOCK_DEBUG
- if (cycles <= 0) {
- printf("xen_microtime: CPU counter has decreased by %" PRId64
- " since last hardclock(9)\n", -cycles);
- }
-#endif
-
-#ifndef XEN3
- /*
- * Work around an intermittent Xen2 bug where, for a period of
- * 1<<32 ns, currently running domains don't get their timer
- * events as usual (and also aren't preempted in favor of
- * other runnable domains). Setting the timer into the past
- * in this way causes it to fire immediately.
- */
- if (cycles > cpu_frequency(ci) / 25 && time_values_up_to_date()) {
- /*
- * 40ms; under Xen2 the timestamp updates and timer ticks
- * should arrive every 10ms.
- */
-#ifdef XEN_CLOCK_DEBUG
- printf("xen_microtime: overlarge TSC offset %llu"
- " (pst=%llu sst=%llu); forcing timer...\n", cycles,
- processed_system_time, shadow_system_time);
-#endif
- HYPERVISOR_set_timer_op(shadow_system_time);
- }
-#endif
- cycles += ci->ci_cc.cc_denom * cpu_frequency(ci) / 1000000000LL;
- tv->tv_usec += cycles * ci->ci_cc.cc_ms_delta * hz / cpu_frequency(ci);
-#ifdef XEN_CLOCK_DEBUG
- if (tv->tv_usec >= 2000000)
- printf("xen_microtime: unexpectedly large tv_usec %ld\n",
tv->tv_usec);
-#endif
- if (tv->tv_usec >= 1000000) {
- tv->tv_sec++;
- tv->tv_usec -= 1000000;
- }
- /* Avoid small backsteps, e.g. at the beginning of a negative
adjustment. */
- if (timerisset(&ci->ci_cc.cc_time) &&
- timercmp(tv, &ci->ci_cc.cc_time, <)) {
- struct timeval backstep;
-
- /* XXXjld: not sure if this check can be safely removed now */
- timersub(&ci->ci_cc.cc_time, tv, &backstep);
- if (backstep.tv_sec == 0) { /* if it was < 1sec */
- *tv = ci->ci_cc.cc_time;
-#ifdef XEN_CLOCK_DEBUG
- printf("xen_microtime[%d]: clamping at %ld.%06ld
(-%ldus)\n",
- (int)ci->ci_cpuid, tv->tv_sec, tv->tv_usec,
backstep.tv_usec);
- } else {
- printf("xen_microtime[%d]: allowing large backstep "
- "%lds to %ld.%06ld\n", (int)ci->ci_cpuid,
- backstep.tv_sec, tv->tv_sec, tv->tv_usec);
-#endif
}
+ splx(s);
}
- ci->ci_cc.cc_time = *tv;
- splx(s);
}
#ifdef DOM0OPS
@@ -463,7 +429,7 @@ xen_timepush(void *arg)
{
struct callout *co = arg;
- resettodr_i();
+ resettodr();
if (xen_timepush_ticks > 0)
callout_schedule(co, xen_timepush_ticks);
}
@@ -496,6 +462,20 @@ sysctl_xen_timepush(SYSCTLFN_ARGS)
}
#endif
+/* ARGSUSED */
+u_int
+xen_get_timecount(struct timecounter *tc)
+{
+ uint64_t ns;
+ int s;
+
+ s = splhigh();
+ ns = get_system_time() - xen_clock_bias;
+ splx(s);
+
+ return (u_int)ns;
+}
+
void
xen_initclocks()
{
@@ -506,6 +486,8 @@ xen_initclocks()
get_time_values_from_xen();
processed_system_time = shadow_system_time;
+ tc_init(&xen_timecounter);
+ /* The splhigh requirements start here. */
event_set_handler(evtch, (int (*)(void *))xen_timer_handler,
NULL, IPL_CLOCK, "clock");
@@ -525,38 +507,37 @@ xen_initclocks()
#endif
}
+/* ARGSUSED */
static int
xen_timer_handler(void *arg, struct intrframe *regs)
{
- int64_t delta, newcc;
- int ticks_done;
- struct timeval oldtime, elapsed;
- struct cpu_info *ci = curcpu();
-
+ int64_t delta;
+ int s, ticks_done;
+
+ s = splhigh();
+#if 0
get_time_values_from_xen();
- newcc = cpu_counter();
+#endif
+ delta = (int64_t)(get_system_time() - processed_system_time);
+ splx(s);
ticks_done = 0;
- delta = (int64_t)(get_system_time() - processed_system_time);
+ /* Several ticks may have passed without our being run; catch up. */
while (delta >= (int64_t)NS_PER_TICK) {
- /* Have hardclock do its thing. */
- oldtime = time;
- hardclock((struct clockframe *)regs);
-
- /* Use that tick length for the coming tick's microtimes. */
- timersub(&time, &oldtime, &elapsed);
-#ifdef XEN_CLOCK_DEBUG
- if (elapsed.tv_sec != 0) {
- printf("xen_timer_handler: hardclock(9) stepped by
%ld.%06lds\n",
- elapsed.tv_sec, elapsed.tv_usec);
- }
-#endif
- ci->ci_cc.cc_ms_delta = elapsed.tv_usec;
-
- delta -= NS_PER_TICK;
+ ++ticks_done;
+ s = splhigh();
processed_system_time += NS_PER_TICK;
- ticks_done++;
+ xen_clock_bias = (delta -= NS_PER_TICK);
+ splx(s);
+ hardclock((struct clockframe *)regs);
}
+
+ if (xen_clock_bias) {
+ s = splhigh();
+ xen_clock_bias = 0;
+ splx(s);
+ }
+
/*
* Re-arm the timer here, if needed; Xen's auto-ticking while runnable
* is useful only for HZ==100, and even then may be out of phase with
@@ -565,14 +546,6 @@ xen_timer_handler(void *arg, struct intr
if (ticks_done != 0)
HYPERVISOR_set_timer_op(processed_system_time + NS_PER_TICK);
- /*
- * Right now, delta holds the number of ns elapsed from when the last
- * hardclock(9) allegedly was to when this domain/vcpu was actually
- * rescheduled.
- */
- ci->ci_cc.cc_denom = delta;
- ci->ci_cc.cc_cc = newcc;
-
return 0;
}
@@ -585,6 +558,7 @@ void
idle_block(void)
{
int s, r;
+
/*
* We set the timer to when we expect the next timer
* interrupt. We could set the timer to later if we could
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