Scott Cheloha <scottchel...@gmail.com> writes:
> On Thu, Jul 28, 2022 at 04:57:41PM -0400, Dave Voutila wrote:
>>
>> Stuart Henderson <s...@spacehopper.org> writes:
>>
>> > On 2022/07/28 12:57, Scott Cheloha wrote:
>> >> On Thu, Jul 28, 2022 at 07:55:40AM -0400, Dave Voutila wrote:
>> >> >
>> >> > This is breaking timecounter selection on my x13 Ryzen 5 Pro laptop
>> >> > running the latest kernel from snaps.
>> >>
>> >> Define "breaking".
>> >
>> > That's clear from the output:
>> >
>> > : On 2022/07/28 07:55, Dave Voutila wrote:
>> > : > $ sysctl -a | grep tsc
>> > : > kern.timecounter.choice=i8254(0) tsc(-1000) acpihpet0(1000)
>> > : > acpitimer0(1000)
>> > : > machdep.tscfreq=2096064730
>> > : > machdep.invarianttsc=1
>> > : >
>> > : > $ sysctl kern.timecounter
>> > : > kern.timecounter.tick=1
>> > : > kern.timecounter.timestepwarnings=0
>> > : > kern.timecounter.hardware=i8254
>> > : > kern.timecounter.choice=i8254(0) tsc(-1000) acpihpet0(1000)
>> > : > acpitimer0(1000)
>> >
>> >> The code detects TSC desync and marks the timecounter non-monotonic.
>> >
>> > That's good (and I think as would have happened before)
>> >
>> >> So it uses the i8254 instead.
>> >
>> > But that's not so good, there are higher prio timecounters available,
>> > acpihpet0 and acpitimer0, which would be better choices than i8254.
>>
>> Exactly my point. Thanks Stuart.
>
> Okay, please try this patch on the machine in question.
That fixes the selection on my x13 gen1; it's choosing acpihpet0 now. No
issue with suspend/resume cycles either.
Also tested the patch on my dual-socket Xeon machine and it looks to
still be properly synchronizing and selecting tsc as with the previous
diff & snapshot kernel.
Is there any special consideration for unhiberate? I can't tell if/when
it is checking the TSCs across the cpus.
>
> It adds a tc_detach() function to kern_tc.c. The first time we fail
> the sync test, the BP calls tc_detach(), changes the TSC's tc_quality
> to a negative value to tell everyone "this is not monotonic", then
> reinstalls the TSC timecounter again with tc_init().
>
> Because we are making this call *once*, from one place, I do not think
> the O(n) removal time matters, so I have not switched the tc_list from
> SLIST to TAILQ.
>
> It is possible for a thread to be asleep in sysctl_tc_hardware()
> during resume, but the thread would be done iterating through the list
> if it had reached rw_enter_write(), so removing/adding tsc_timecounter
> to the list during resume cannot break list traversal.
>
> Switching the active timecounter during resume is also fine. The only
> race is with tc_adjfreq(). If a thread is asleep in adjfreq(2) when
> the system suspends, and we change the active timecounter during
> resume, the frequency change may be applied to the "wrong" timecounter.
>
> ... but this is always a race, because adjfreq(2) only operates on the
> active timecounter, and root can change it at any time via sysctl(2).
> So it's not a new problem.
>
> ...
>
> It might be simpler to just change tc_lock from a rwlock to a mutex.
> Then the MP analysis is much simpler across a suspend/resume.
>
> Index: sys/arch/amd64/amd64/tsc.c
> ===================================================================
> RCS file: /cvs/src/sys/arch/amd64/amd64/tsc.c,v
> retrieving revision 1.24
> diff -u -p -r1.24 tsc.c
> --- sys/arch/amd64/amd64/tsc.c 31 Aug 2021 15:11:54 -0000 1.24
> +++ sys/arch/amd64/amd64/tsc.c 29 Jul 2022 01:06:17 -0000
> @@ -36,13 +36,6 @@ int tsc_recalibrate;
> uint64_t tsc_frequency;
> int tsc_is_invariant;
>
> -#define TSC_DRIFT_MAX 250
> -#define TSC_SKEW_MAX 100
> -int64_t tsc_drift_observed;
> -
> -volatile int64_t tsc_sync_val;
> -volatile struct cpu_info *tsc_sync_cpu;
> -
> u_int tsc_get_timecount(struct timecounter *tc);
> void tsc_delay(int usecs);
>
> @@ -236,22 +229,12 @@ cpu_recalibrate_tsc(struct timecounter *
> u_int
> tsc_get_timecount(struct timecounter *tc)
> {
> - return rdtsc_lfence() + curcpu()->ci_tsc_skew;
> + return rdtsc_lfence();
> }
>
> void
> tsc_timecounter_init(struct cpu_info *ci, uint64_t cpufreq)
> {
> -#ifdef TSC_DEBUG
> - printf("%s: TSC skew=%lld observed drift=%lld\n", ci->ci_dev->dv_xname,
> - (long long)ci->ci_tsc_skew, (long long)tsc_drift_observed);
> -#endif
> - if (ci->ci_tsc_skew < -TSC_SKEW_MAX || ci->ci_tsc_skew > TSC_SKEW_MAX) {
> - printf("%s: disabling user TSC (skew=%lld)\n",
> - ci->ci_dev->dv_xname, (long long)ci->ci_tsc_skew);
> - tsc_timecounter.tc_user = 0;
> - }
> -
> if (!(ci->ci_flags & CPUF_PRIMARY) ||
> !(ci->ci_flags & CPUF_CONST_TSC) ||
> !(ci->ci_flags & CPUF_INVAR_TSC))
> @@ -268,111 +251,276 @@ tsc_timecounter_init(struct cpu_info *ci
> calibrate_tsc_freq();
> }
>
> - if (tsc_drift_observed > TSC_DRIFT_MAX) {
> - printf("ERROR: %lld cycle TSC drift observed\n",
> - (long long)tsc_drift_observed);
> - tsc_timecounter.tc_quality = -1000;
> - tsc_timecounter.tc_user = 0;
> - tsc_is_invariant = 0;
> - }
> -
> tc_init(&tsc_timecounter);
> }
>
> -/*
> - * Record drift (in clock cycles). Called during AP startup.
> - */
> void
> -tsc_sync_drift(int64_t drift)
> +tsc_delay(int usecs)
> {
> - if (drift < 0)
> - drift = -drift;
> - if (drift > tsc_drift_observed)
> - tsc_drift_observed = drift;
> + uint64_t interval, start;
> +
> + interval = (uint64_t)usecs * tsc_frequency / 1000000;
> + start = rdtsc_lfence();
> + while (rdtsc_lfence() - start < interval)
> + CPU_BUSY_CYCLE();
> }
>
> +#ifdef MULTIPROCESSOR
> +
> +#define TSC_DEBUG 1
> +
> +/*
> + * Protections for global variables in this code:
> + *
> + * a Modified atomically
> + * b Protected by a barrier
> + * p Only modified by the primary CPU
> + */
> +
> +#define TSC_TEST_MS 1 /* Test round duration */
> +#define TSC_TEST_ROUNDS 2 /* Number of test rounds */
> +
> /*
> - * Called during startup of APs, by the boot processor. Interrupts
> - * are disabled on entry.
> + * tsc_test_status.val is isolated to its own cache line to limit
> + * false sharing and reduce the test's margin of error.
> */
> +struct tsc_test_status {
> + volatile uint64_t val; /* [b] Latest RDTSC value */
> + uint64_t pad1[7];
> + uint64_t lag_count; /* [b] Number of lags seen by CPU */
> + uint64_t lag_max; /* [b] Biggest lag seen by CPU */
> + int64_t adj; /* [b] Initial IA32_TSC_ADJUST value */
> + uint64_t pad2[5];
> +} __aligned(64);
> +struct tsc_test_status tsc_ap_status; /* [b] Test results from AP */
> +struct tsc_test_status tsc_bp_status; /* [p] Test results from BP */
> +uint64_t tsc_test_cycles; /* [p] TSC cycles per test round */
> +const char *tsc_ap_name; /* [b] Name of AP running test */
> +volatile u_int tsc_egress_barrier; /* [a] Test end barrier */
> +volatile u_int tsc_ingress_barrier; /* [a] Test start barrier */
> +volatile u_int tsc_test_rounds; /* [p] Remaining test rounds */
> +int tsc_is_synchronized = 1; /* [p] TSC sync'd across all CPUs? */
> +
> +void tsc_report_test_results(void);
> +void tsc_reset_adjust(struct tsc_test_status *);
> +void tsc_test_ap(void);
> +void tsc_test_bp(void);
> +
> void
> -tsc_read_bp(struct cpu_info *ci, uint64_t *bptscp, uint64_t *aptscp)
> +tsc_test_sync_bp(struct cpu_info *ci)
> {
> - uint64_t bptsc;
> -
> - if (atomic_swap_ptr(&tsc_sync_cpu, ci) != NULL)
> - panic("tsc_sync_bp: 1");
> + if (!tsc_is_invariant)
> + return;
> +#ifndef TSC_DEBUG
> + /* No point in testing again if we already failed. */
> + if (!tsc_is_synchronized)
> + return;
> +#endif
> + /* Reset IA32_TSC_ADJUST if it exists. */
> + tsc_reset_adjust(&tsc_bp_status);
>
> - /* Flag it and read our TSC. */
> - atomic_setbits_int(&ci->ci_flags, CPUF_SYNCTSC);
> - bptsc = (rdtsc_lfence() >> 1);
> + /* Reset the test cycle limit and round count. */
> + tsc_test_cycles = TSC_TEST_MS * tsc_frequency / 1000;
> + tsc_test_rounds = TSC_TEST_ROUNDS;
> +
> + do {
> + /*
> + * Pass through the ingress barrier, run the test,
> + * then wait for the AP to reach the egress barrier.
> + */
> + atomic_inc_int(&tsc_ingress_barrier);
> + while (tsc_ingress_barrier != 2)
> + CPU_BUSY_CYCLE();
> + tsc_test_bp();
> + while (tsc_egress_barrier != 1)
> + CPU_BUSY_CYCLE();
> +
> + /*
> + * Report what happened. Adjust the TSC's tc_quality
> + * if this is the first time we've failed the test.
> + */
> + tsc_report_test_results();
> + if (tsc_ap_status.lag_count || tsc_bp_status.lag_count) {
> + if (tsc_is_synchronized) {
> + tsc_is_synchronized = 0;
> + tc_detach(&tsc_timecounter);
> + tsc_timecounter.tc_quality = -1000;
> + tc_init(&tsc_timecounter);
> + }
> + tsc_test_rounds = 0;
> + } else
> + tsc_test_rounds--;
> +
> + /*
> + * Clean up for the next round. It is safe to reset the
> + * ingress barrier because at this point we know the AP
> + * has reached the egress barrier.
> + */
> + memset(&tsc_ap_status, 0, sizeof tsc_ap_status);
> + memset(&tsc_bp_status, 0, sizeof tsc_bp_status);
> + tsc_ingress_barrier = 0;
> + if (tsc_test_rounds == 0)
> + tsc_ap_name = NULL;
> +
> + /*
> + * Pass through the egress barrier and release the AP.
> + * The AP is responsible for resetting the egress barrier.
> + */
> + if (atomic_inc_int_nv(&tsc_egress_barrier) != 2)
> + panic("%s: unexpected egress count", __func__);
> + } while (tsc_test_rounds > 0);
> +}
>
> - /* Wait for remote to complete, and read ours again. */
> - while ((ci->ci_flags & CPUF_SYNCTSC) != 0)
> - membar_consumer();
> - bptsc += (rdtsc_lfence() >> 1);
> +void
> +tsc_test_sync_ap(struct cpu_info *ci)
> +{
> + if (!tsc_is_invariant)
> + return;
> +#ifndef TSC_DEBUG
> + if (!tsc_is_synchronized)
> + return;
> +#endif
> + /* The BP needs our name in order to report any problems. */
> + if (atomic_cas_ptr(&tsc_ap_name, NULL, ci->ci_dev->dv_xname) != NULL) {
> + panic("%s: %s: tsc_ap_name is not NULL: %s",
> + __func__, ci->ci_dev->dv_xname, tsc_ap_name);
> + }
>
> - /* Wait for the results to come in. */
> - while (tsc_sync_cpu == ci)
> - CPU_BUSY_CYCLE();
> - if (tsc_sync_cpu != NULL)
> - panic("tsc_sync_bp: 2");
> + tsc_reset_adjust(&tsc_ap_status);
>
> - *bptscp = bptsc;
> - *aptscp = tsc_sync_val;
> + /*
> + * The AP is only responsible for running the test and
> + * resetting the egress barrier. The BP handles everything
> + * else.
> + */
> + do {
> + atomic_inc_int(&tsc_ingress_barrier);
> + while (tsc_ingress_barrier != 2)
> + CPU_BUSY_CYCLE();
> + tsc_test_ap();
> + atomic_inc_int(&tsc_egress_barrier);
> + while (atomic_cas_uint(&tsc_egress_barrier, 2, 0) != 2)
> + CPU_BUSY_CYCLE();
> + } while (tsc_test_rounds > 0);
> }
>
> void
> -tsc_sync_bp(struct cpu_info *ci)
> +tsc_report_test_results(void)
> {
> - uint64_t bptsc, aptsc;
> + u_int round = TSC_TEST_ROUNDS - tsc_test_rounds + 1;
>
> - tsc_read_bp(ci, &bptsc, &aptsc); /* discarded - cache effects */
> - tsc_read_bp(ci, &bptsc, &aptsc);
> -
> - /* Compute final value to adjust for skew. */
> - ci->ci_tsc_skew = bptsc - aptsc;
> + if (tsc_bp_status.adj != 0) {
> + printf("tsc: cpu0: IA32_TSC_ADJUST: %lld -> 0\n",
> + tsc_bp_status.adj);
> + }
> + if (tsc_ap_status.adj != 0) {
> + printf("tsc: %s: IA32_TSC_ADJUST: %lld -> 0\n",
> + tsc_ap_name, tsc_ap_status.adj);
> + }
> + if (tsc_ap_status.lag_count > 0 || tsc_bp_status.lag_count > 0) {
> + printf("tsc: cpu0/%s: sync test round %u/%u failed\n",
> + tsc_ap_name, round, TSC_TEST_ROUNDS);
> + }
> + if (tsc_bp_status.lag_count > 0) {
> + printf("tsc: cpu0/%s: cpu0: %llu lags %llu cycles\n",
> + tsc_ap_name, tsc_bp_status.lag_count,
> + tsc_bp_status.lag_max);
> + }
> + if (tsc_ap_status.lag_count > 0) {
> + printf("tsc: cpu0/%s: %s: %llu lags %llu cycles\n",
> + tsc_ap_name, tsc_ap_name, tsc_ap_status.lag_count,
> + tsc_ap_status.lag_max);
> + }
> }
>
> /*
> - * Called during startup of AP, by the AP itself. Interrupts are
> - * disabled on entry.
> + * Reset IA32_TSC_ADJUST if we have it.
> + *
> + * XXX We should rearrange cpu_hatch() so that the feature
> + * flags are already set before we get here. Check CPUID
> + * by hand until then.
> */
> void
> -tsc_post_ap(struct cpu_info *ci)
> +tsc_reset_adjust(struct tsc_test_status *tts)
> {
> - uint64_t tsc;
> -
> - /* Wait for go-ahead from primary. */
> - while ((ci->ci_flags & CPUF_SYNCTSC) == 0)
> - membar_consumer();
> - tsc = (rdtsc_lfence() >> 1);
> -
> - /* Instruct primary to read its counter. */
> - atomic_clearbits_int(&ci->ci_flags, CPUF_SYNCTSC);
> - tsc += (rdtsc_lfence() >> 1);
> -
> - /* Post result. Ensure the whole value goes out atomically. */
> - (void)atomic_swap_64(&tsc_sync_val, tsc);
> + uint32_t eax, ebx, ecx, edx;
>
> - if (atomic_swap_ptr(&tsc_sync_cpu, NULL) != ci)
> - panic("tsc_sync_ap");
> + CPUID(0, eax, ebx, ecx, edx);
> + if (eax >= 7) {
> + CPUID_LEAF(7, 0, eax, ebx, ecx, edx);
> + if (ISSET(ebx, SEFF0EBX_TSC_ADJUST)) {
> + tts->adj = rdmsr(MSR_TSC_ADJUST);
> + if (tts->adj != 0)
> + wrmsr(MSR_TSC_ADJUST, 0);
> + }
> + }
> }
>
> void
> -tsc_sync_ap(struct cpu_info *ci)
> +tsc_test_ap(void)
> {
> - tsc_post_ap(ci);
> - tsc_post_ap(ci);
> + uint64_t ap_val, bp_val, end, lag;
> + u_int i = 0;
> +
> + ap_val = rdtsc_lfence();
> + end = ap_val + tsc_test_cycles;
> + while (ap_val < end) {
> + /*
> + * The LFENCE ensures bp_val predates ap_val. If ap_val
> + * is smaller than bp_val then the AP's TSC must lag that
> + * of the BP and the counters cannot be synchronized.
> + */
> + bp_val = tsc_bp_status.val;
> + ap_val = rdtsc_lfence();
> + tsc_ap_status.val = ap_val;
> +
> + /*
> + * Ensure the other CPU has a chance to run. This is
> + * crucial if the other CPU is our SMT sibling. SMT
> + * starvation can prevent this test from detecting
> + * relatively large lags. Eight is an arbitrary value,
> + * but it seems to work in practice without impacting
> + * the sensitivity of the test for non-sibling threads.
> + */
> + if (++i % 8 == 0)
> + CPU_BUSY_CYCLE();
> +
> + /*
> + * Record the magnitude of the problem if our TSC lags
> + * the other.
> + */
> + if (ap_val < bp_val) {
> + tsc_ap_status.lag_count++;
> + lag = bp_val - ap_val;
> + if (tsc_ap_status.lag_max < lag)
> + tsc_ap_status.lag_max = lag;
> + }
> + }
> }
>
> void
> -tsc_delay(int usecs)
> +tsc_test_bp(void)
> {
> - uint64_t interval, start;
> + uint64_t ap_val, bp_val, end, lag;
> + u_int i = 0;
>
> - interval = (uint64_t)usecs * tsc_frequency / 1000000;
> - start = rdtsc_lfence();
> - while (rdtsc_lfence() - start < interval)
> - CPU_BUSY_CYCLE();
> + bp_val = rdtsc_lfence();
> + end = bp_val + tsc_test_cycles;
> + while (bp_val < end) {
> + ap_val = tsc_ap_status.val;
> + bp_val = rdtsc_lfence();
> + tsc_bp_status.val = bp_val;
> +
> + if (++i % 8 == 0)
> + CPU_BUSY_CYCLE();
> +
> + if (bp_val < ap_val) {
> + tsc_bp_status.lag_count++;
> + lag = ap_val - bp_val;
> + if (tsc_bp_status.lag_max < lag)
> + tsc_bp_status.lag_max = lag;
> + }
> + }
> }
> +
> +#endif /* MULTIPROCESSOR */
> Index: sys/arch/amd64/amd64/cpu.c
> ===================================================================
> RCS file: /cvs/src/sys/arch/amd64/amd64/cpu.c,v
> retrieving revision 1.156
> diff -u -p -r1.156 cpu.c
> --- sys/arch/amd64/amd64/cpu.c 26 Apr 2022 08:35:30 -0000 1.156
> +++ sys/arch/amd64/amd64/cpu.c 29 Jul 2022 01:06:17 -0000
> @@ -772,9 +772,9 @@ cpu_init(struct cpu_info *ci)
> lcr4(cr4 & ~CR4_PGE);
> lcr4(cr4);
>
> - /* Synchronize TSC */
> + /* Check if TSC is synchronized. */
> if (cold && !CPU_IS_PRIMARY(ci))
> - tsc_sync_ap(ci);
> + tsc_test_sync_ap(ci);
> #endif
> }
>
> @@ -854,18 +854,14 @@ cpu_start_secondary(struct cpu_info *ci)
> #endif
> } else {
> /*
> - * Synchronize time stamp counters. Invalidate cache and
> - * synchronize twice (in tsc_sync_bp) to minimize possible
> - * cache effects. Disable interrupts to try and rule out any
> - * external interference.
> + * Test if TSCs are synchronized. Invalidate cache to
> + * minimize possible cache effects. Disable interrupts to
> + * try to rule out external interference.
> */
> s = intr_disable();
> wbinvd();
> - tsc_sync_bp(ci);
> + tsc_test_sync_bp(ci);
> intr_restore(s);
> -#ifdef TSC_DEBUG
> - printf("TSC skew=%lld\n", (long long)ci->ci_tsc_skew);
> -#endif
> }
>
> if ((ci->ci_flags & CPUF_IDENTIFIED) == 0) {
> @@ -890,7 +886,6 @@ void
> cpu_boot_secondary(struct cpu_info *ci)
> {
> int i;
> - int64_t drift;
> u_long s;
>
> atomic_setbits_int(&ci->ci_flags, CPUF_GO);
> @@ -905,18 +900,11 @@ cpu_boot_secondary(struct cpu_info *ci)
> db_enter();
> #endif
> } else if (cold) {
> - /* Synchronize TSC again, check for drift. */
> - drift = ci->ci_tsc_skew;
> + /* Test if TSCs are synchronized again. */
> s = intr_disable();
> wbinvd();
> - tsc_sync_bp(ci);
> + tsc_test_sync_bp(ci);
> intr_restore(s);
> - drift -= ci->ci_tsc_skew;
> -#ifdef TSC_DEBUG
> - printf("TSC skew=%lld drift=%lld\n",
> - (long long)ci->ci_tsc_skew, (long long)drift);
> -#endif
> - tsc_sync_drift(drift);
> }
> }
>
> @@ -942,13 +930,12 @@ cpu_hatch(void *v)
> #endif
>
> /*
> - * Synchronize the TSC for the first time. Note that interrupts are
> - * off at this point.
> + * Test if our TSC is synchronized for the first time.
> + * Note that interrupts are off at this point.
> */
> wbinvd();
> ci->ci_flags |= CPUF_PRESENT;
> - ci->ci_tsc_skew = 0; /* reset on resume */
> - tsc_sync_ap(ci);
> + tsc_test_sync_ap(ci);
>
> lapic_enable();
> lapic_startclock();
> Index: sys/arch/amd64/include/cpu.h
> ===================================================================
> RCS file: /cvs/src/sys/arch/amd64/include/cpu.h,v
> retrieving revision 1.145
> diff -u -p -r1.145 cpu.h
> --- sys/arch/amd64/include/cpu.h 12 Jul 2022 04:46:00 -0000 1.145
> +++ sys/arch/amd64/include/cpu.h 29 Jul 2022 01:06:18 -0000
> @@ -207,8 +207,6 @@ struct cpu_info {
> paddr_t ci_vmxon_region_pa;
> struct vmxon_region *ci_vmxon_region;
>
> - int64_t ci_tsc_skew; /* counter skew vs cpu0 */
> -
> char ci_panicbuf[512];
>
> paddr_t ci_vmcs_pa;
> @@ -228,7 +226,6 @@ struct cpu_info {
> #define CPUF_INVAR_TSC 0x0100 /* CPU has invariant TSC */
> #define CPUF_USERXSTATE 0x0200 /* CPU has curproc's xsave
> state */
>
> -#define CPUF_SYNCTSC 0x0800 /* Synchronize TSC */
> #define CPUF_PRESENT 0x1000 /* CPU is present */
> #define CPUF_RUNNING 0x2000 /* CPU is running */
> #define CPUF_PAUSE 0x4000 /* CPU is paused in DDB */
> Index: sys/arch/amd64/include/cpuvar.h
> ===================================================================
> RCS file: /cvs/src/sys/arch/amd64/include/cpuvar.h,v
> retrieving revision 1.11
> diff -u -p -r1.11 cpuvar.h
> --- sys/arch/amd64/include/cpuvar.h 16 May 2021 04:33:05 -0000 1.11
> +++ sys/arch/amd64/include/cpuvar.h 29 Jul 2022 01:06:18 -0000
> @@ -97,8 +97,7 @@ void identifycpu(struct cpu_info *);
> void cpu_init(struct cpu_info *);
> void cpu_init_first(void);
>
> -void tsc_sync_drift(int64_t);
> -void tsc_sync_bp(struct cpu_info *);
> -void tsc_sync_ap(struct cpu_info *);
> +void tsc_test_sync_bp(struct cpu_info *);
> +void tsc_test_sync_ap(struct cpu_info *);
>
> #endif
> Index: sys/sys/timetc.h
> ===================================================================
> RCS file: /cvs/src/sys/sys/timetc.h,v
> retrieving revision 1.12
> diff -u -p -r1.12 timetc.h
> --- sys/sys/timetc.h 6 Jul 2020 13:33:09 -0000 1.12
> +++ sys/sys/timetc.h 29 Jul 2022 01:06:18 -0000
> @@ -120,6 +120,7 @@ extern struct timekeep *timekeep;
> u_int64_t tc_getfrequency(void);
> u_int64_t tc_getprecision(void);
> void tc_init(struct timecounter *tc);
> +void tc_detach(struct timecounter *);
> void tc_setclock(const struct timespec *ts);
> void tc_setrealtimeclock(const struct timespec *ts);
> void tc_ticktock(void);
> Index: sys/kern/kern_tc.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/kern_tc.c,v
> retrieving revision 1.76
> diff -u -p -r1.76 kern_tc.c
> --- sys/kern/kern_tc.c 23 Jul 2022 22:58:51 -0000 1.76
> +++ sys/kern/kern_tc.c 29 Jul 2022 01:06:18 -0000
> @@ -458,6 +458,36 @@ tc_init(struct timecounter *tc)
> timecounter = tc;
> }
>
> +/*
> + * Remove the given timecounter from the list of counters.
> + * Activate the best remaining counter if it was the active
> + * counter.
> + */
> +void
> +tc_detach(struct timecounter *tc)
> +{
> + struct timecounter *best = &dummy_timecounter, *tmp;
> +
> + if (tc == &dummy_timecounter)
> + panic("%s: cannot detach dummy counter", __func__);
> +
> + SLIST_REMOVE(&tc_list, tc, timecounter, tc_next);
> + if (timecounter == tc) {
> + SLIST_FOREACH(tmp, &tc_list, tc_next) {
> + if (tmp->tc_quality < 0)
> + continue;
> + if (tmp->tc_quality < best->tc_quality)
> + continue;
> + if (tmp->tc_quality == best->tc_quality &&
> + tmp->tc_frequency < best->tc_frequency)
> + continue;
> + best = tmp;
> + }
> + enqueue_randomness(best->tc_get_timecount(best));
> + timecounter = best;
> + }
> +}
> +
> /* Report the frequency of the current timecounter. */
> u_int64_t
> tc_getfrequency(void)
