Re: [PATCH] KVM: selftests: Compare wall time from xen shinfo against KVM_GET_CLOCK

From: Vitaly Kuznetsov
Date: Mon Jan 29 2024 - 04:05:09 EST

Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> writes:

> xen_shinfo_test is observed to be flaky failing sporadically with
> "VM time too old". With min_ts/max_ts debug print added:
>
> Wall clock (v 3269818) 1704906491.986255664
> Time info 1: v 1282712 tsc 33530585736 time 14014430025 mul 3587552223 shift 4294967295 flags 1
> Time info 2: v 1282712 tsc 33530585736 time 14014430025 mul 3587552223 shift 4294967295 flags 1
> min_ts: 1704906491.986312153
> max_ts: 1704906506.001006963
> ==== Test Assertion Failure ====
> x86_64/xen_shinfo_test.c:1003: cmp_timespec(&min_ts, &vm_ts) <= 0
> pid=32724 tid=32724 errno=4 - Interrupted system call
> 1 0x00000000004030ad: main at xen_shinfo_test.c:1003
> 2 0x00007fca6b23feaf: ?? ??:0
> 3 0x00007fca6b23ff5f: ?? ??:0
> 4 0x0000000000405e04: _start at ??:?
> VM time too old
>
> The test compares wall clock data from shinfo (which is the output of
> kvm_get_wall_clock_epoch()) against clock_gettime(CLOCK_REALTIME) in the
> host system before the VM is created. In the example above, it compares
>
> shinfo: 1704906491.986255664 vs min_ts: 1704906491.986312153
>
> and fails as the later is greater than the former. While this sounds like
> a sane test, it doesn't pass reality check: kvm_get_wall_clock_epoch()
> calculates guest's epoch (realtime when the guest was created) by
> subtracting kvmclock from the current realtime and the calculation happens
> when shinfo is setup. The problem is that kvmclock is a raw clock and
> realtime clock is affected by NTP. This means that if realtime ticks with a
> slightly reduced frequency, "guest's epoch" calculated by
> kvm_get_wall_clock_epoch() will actually tick backwards! This is not a big
> issue from guest's perspective as the guest can't really observe this but
> this epoch can't be compared with a fixed clock_gettime() on the host.
>
> Replace the check with comparing wall clock data from shinfo to
> KVM_GET_CLOCK. The later gives both realtime and kvmclock so guest's epoch
> can be calculated by subtraction. Note, the computed epoch may still differ
> a few nanoseconds from shinfo as different TSC is used and there are
> rounding errors but 100 nanoseconds margin should be enough to cover
> it (famous last words).
>
> Reported-by: Jan Richter <jarichte@xxxxxxxxxx>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
> .../selftests/kvm/x86_64/xen_shinfo_test.c | 36 ++++++++-----------
> 1 file changed, 14 insertions(+), 22 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c b/tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c
> index 9ec9ab60b63e..5e1ad243d95d 100644
> --- a/tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c
> @@ -375,20 +375,6 @@ static void guest_code(void)
> GUEST_SYNC(TEST_DONE);
> }
>
> -static int cmp_timespec(struct timespec *a, struct timespec *b)
> -{
> - if (a->tv_sec > b->tv_sec)
> - return 1;
> - else if (a->tv_sec < b->tv_sec)
> - return -1;
> - else if (a->tv_nsec > b->tv_nsec)
> - return 1;
> - else if (a->tv_nsec < b->tv_nsec)
> - return -1;
> - else
> - return 0;
> -}
> -
> static struct vcpu_info *vinfo;
> static struct kvm_vcpu *vcpu;
>
> @@ -425,7 +411,6 @@ static void *juggle_shinfo_state(void *arg)
>
> int main(int argc, char *argv[])
> {
> - struct timespec min_ts, max_ts, vm_ts;
> struct kvm_xen_hvm_attr evt_reset;
> struct kvm_vm *vm;
> pthread_t thread;
> @@ -443,8 +428,6 @@ int main(int argc, char *argv[])
> bool do_eventfd_tests = !!(xen_caps & KVM_XEN_HVM_CONFIG_EVTCHN_2LEVEL);
> bool do_evtchn_tests = do_eventfd_tests && !!(xen_caps & KVM_XEN_HVM_CONFIG_EVTCHN_SEND);
>
> - clock_gettime(CLOCK_REALTIME, &min_ts);
> -
> vm = vm_create_with_one_vcpu(&vcpu, guest_code);
>
> /* Map a region for the shared_info page */
> @@ -969,7 +952,6 @@ int main(int argc, char *argv[])
> vm_ioctl(vm, KVM_XEN_HVM_SET_ATTR, &evt_reset);
>
> alarm(0);
> - clock_gettime(CLOCK_REALTIME, &max_ts);
>
> /*
> * Just a *really* basic check that things are being put in the
> @@ -978,11 +960,16 @@ int main(int argc, char *argv[])
> */
> struct pvclock_wall_clock *wc;
> struct pvclock_vcpu_time_info *ti, *ti2;
> + struct kvm_clock_data kcdata;
> + long long delta;
>
> wc = addr_gpa2hva(vm, SHINFO_REGION_GPA + 0xc00);
> ti = addr_gpa2hva(vm, SHINFO_REGION_GPA + 0x40 + 0x20);
> ti2 = addr_gpa2hva(vm, PVTIME_ADDR);
>
> + vm_ioctl(vm, KVM_GET_CLOCK, &kcdata);
> + delta = (wc->sec * NSEC_PER_SEC + wc->nsec) - (kcdata.realtime - kcdata.clock);
> +
> if (verbose) {
> printf("Wall clock (v %d) %d.%09d\n", wc->version, wc->sec, wc->nsec);
> printf("Time info 1: v %u tsc %" PRIu64 " time %" PRIu64 " mul %u shift %u flags %x\n",
> @@ -991,14 +978,19 @@ int main(int argc, char *argv[])
> printf("Time info 2: v %u tsc %" PRIu64 " time %" PRIu64 " mul %u shift %u flags %x\n",
> ti2->version, ti2->tsc_timestamp, ti2->system_time, ti2->tsc_to_system_mul,
> ti2->tsc_shift, ti2->flags);
> + printf("KVM_GET_CLOCK realtime: %lld.%09lld\n", kcdata.realtime / NSEC_PER_SEC,
> + kcdata.realtime % NSEC_PER_SEC);
> + printf("KVM_GET_CLOCK clock: %lld.%09lld\n", kcdata.clock / NSEC_PER_SEC,
> + kcdata.clock % NSEC_PER_SEC);
> }
>
> - vm_ts.tv_sec = wc->sec;
> - vm_ts.tv_nsec = wc->nsec;
> TEST_ASSERT(wc->version && !(wc->version & 1),
> "Bad wallclock version %x", wc->version);
> - TEST_ASSERT(cmp_timespec(&min_ts, &vm_ts) <= 0, "VM time too old");
> - TEST_ASSERT(cmp_timespec(&max_ts, &vm_ts) >= 0, "VM time too new");
> +
> + TEST_ASSERT(llabs(delta) < 100,
> + "Guest's epoch from shinfo %d.%09d differs from KVM_GET_CLOCK %lld.%lld",
> + wc->sec, wc->nsec, (kcdata.realtime - kcdata.clock) / NSEC_PER_SEC,
> + (kcdata.realtime - kcdata.clock) % NSEC_PER_SEC);
>
> TEST_ASSERT(ti->version && !(ti->version & 1),
> "Bad time_info version %x", ti->version);

Ping?

--
Vitaly