[Bug 216177] kvm-unit-tests vmx has about 60% of failure chance

bugzilla-daemon
2022-06-29T02:50:08Z

--- Comment #13 from Yang Lixiao (lixiao.yang@intel.com) ---
(In reply to Jim Mattson from comment #12)
> On Tue, Jun 28, 2022 at 5:22 PM <bugzilla-daemon@kernel.org> wrote:
> >
> > https://bugzilla.kernel.org/show_bug.cgi?id=216177
> >
> > --- Comment #11 from Yang Lixiao (lixiao.yang@intel.com) ---
> > (In reply to Jim Mattson from comment #10)
> > > On Mon, Jun 27, 2022 at 11:32 PM <bugzilla-daemon@kernel.org> wrote:
> > > >
> > > > https://bugzilla.kernel.org/show_bug.cgi?id=216177
> > > >
> > > > --- Comment #9 from Yang Lixiao (lixiao.yang@intel.com) ---
> > > > (In reply to Jim Mattson from comment #8)
> > > > > On Mon, Jun 27, 2022 at 8:54 PM Nadav Amit <nadav.amit@gmail.com>
> > wrote:
> > > > >
> > > > > > The failure on bare-metal that I experienced hints that this is
> > either
> > > a
> > > > > test
> > > > > > bug or (much less likely) a hardware bug. But I do not think it is
> > > likely
> > > > > to
> > > > > > be
> > > > > > a KVM bug.
> > > > >
> > > > > KVM does not use the VMX-preemption timer to virtualize L1's
> > > > > VMX-preemption timer (and that is why KVM is broken). The KVM bug was
> > > > > introduced with commit f4124500c2c1 ("KVM: nVMX: Fully emulate
> > > > > preemption timer"), which uses an L0 CLOCK_MONOTONIC hrtimer to
> > > > > emulate L1's VMX-preemption timer. There are many reasons that this
> > > > > cannot possibly work, not the least of which is that the
> > > > > CLOCK_MONOTONIC timer is subject to time slew.
> > > > >
> > > > > Currently, KVM reserves L0's VMX-preemption timer for emulating L1's
> > > > > APIC timer. Better would be to determine whether L1's APIC timer or
> > > > > L1's VMX-preemption timer is scheduled to fire first, and use L0's
> > > > > VMX-preemption timer to trigger a VM-exit on the nearest alarm.
> > > > > Alternatively, as Sean noted, one could perhaps arrange for the
> > > > > hrtimer to fire early enough that it won't fire late, but I don't
> > > > > really think that's a viable solution.
> > > > >
> > > > > I can't explain the bare-metal failures, but I will note that the
> test
> > > > > assumes the default treatment of SMIs and SMM. The test will likely
> > > > > fail with the dual-monitor treatment of SMIs and SMM. Aside from the
> > > > > older CPUs with broken VMX-preemption timers, I don't know of any
> > > > > relevant errata.
> > > > >
> > > > > Of course, it is possible that the test itself is buggy. For the
> > > > > person who reported bare-metal failures on Ice Lake and Cooper Lake,
> > > > > how long was the test in VMX non-root mode past the VMX-preemption
> > > > > timer deadline?
> > > >
> > > > On the first Ice lake:
> > > > Test suite: vmx_preemption_timer_expiry_test
> > > > FAIL: Last stored guest TSC (28067103426) < TSC deadline (28067086048)
> > > >
> > > > On the second Ice lake:
> > > > Test suite: vmx_preemption_timer_expiry_test
> > > > FAIL: Last stored guest TSC (27014488614) < TSC deadline (27014469152)
> > > >
> > > > On Cooper lake:
> > > > Test suite: vmx_preemption_timer_expiry_test
> > > > FAIL: Last stored guest TSC (29030585690) < TSC deadline (29030565024)
> > >
> > > Wow! Those are *huge* overruns. What is the value of MSR 0x9B on these
> > hosts?
> >
> > All of the values of MSR 0x9B on the three hosts are 0.
> >
> > --
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> Doh! There is a glaring bug in the test. I'll post a fix soon.

Thanks!