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

[Sean Christopherson]

On Mon, Jan 29, 2024, Vitaly Kuznetsov wrote:
> 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>
> > ---

David, any objection?