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KVM: selftests: Compare wall time from xen shinfo against KVM_GET_CLOCK

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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, CLOCK_REALTIME is susceptible to
leap seconds jumps but there's no better alternative in KVM at this
moment. Leave a comment and accept 1s delta.

Reported-by: Jan Richter <jarichte@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240206151950.31174-1-vkuznets@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Vitaly Kuznetsov 2024-02-06 16:19:50 +01:00 committed by Sean Christopherson
parent d85465f277
commit 201142d160
1 changed files with 28 additions and 22 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

50
tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c
View File

@ -380,20 +380,6 @@ wait_for_timer:
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 shared_info *shinfo;
static struct vcpu_info *vinfo;
static struct kvm_vcpu *vcpu;
@ -449,7 +435,6 @@ 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;
@ -468,8 +453,6 @@ int main(int argc, char *argv[])
bool do_evtchn_tests = do_eventfd_tests && !!(xen_caps & KVM_XEN_HVM_CONFIG_EVTCHN_SEND);
bool has_shinfo_hva = !!(xen_caps & KVM_XEN_HVM_CONFIG_SHARED_INFO_HVA);
clock_gettime(CLOCK_REALTIME, &min_ts);
vm = vm_create_with_one_vcpu(&vcpu, guest_code);
/* Map a region for the shared_info page */
@ -1010,7 +993,6 @@ 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
@ -1019,6 +1001,8 @@ 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);
@ -1034,12 +1018,34 @@ int main(int argc, char *argv[])
ti2->tsc_shift, ti2->flags);
}
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");
vm_ioctl(vm, KVM_GET_CLOCK, &kcdata);
if (kcdata.flags & KVM_CLOCK_REALTIME) {
if (verbose) {
printf("KVM_GET_CLOCK clock: %lld.%09lld\n",
kcdata.clock / NSEC_PER_SEC, kcdata.clock % NSEC_PER_SEC);
printf("KVM_GET_CLOCK realtime: %lld.%09lld\n",
kcdata.realtime / NSEC_PER_SEC, kcdata.realtime % NSEC_PER_SEC);
}
delta = (wc->sec * NSEC_PER_SEC + wc->nsec) - (kcdata.realtime - kcdata.clock);
/*
* KVM_GET_CLOCK gives CLOCK_REALTIME which jumps on leap seconds updates but
* unfortunately KVM doesn't currently offer a CLOCK_TAI alternative. Accept 1s
* delta as testing clock accuracy is not the goal here. The test just needs to
* check that the value in shinfo is somewhat sane.
*/
TEST_ASSERT(llabs(delta) < NSEC_PER_SEC,
"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);
} else {
pr_info("Missing KVM_CLOCK_REALTIME, skipping shinfo epoch sanity check\n");
}
TEST_ASSERT(ti->version && !(ti->version & 1),
"Bad time_info version %x", ti->version);