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

[bugzilla-daemon@kernel.org]

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)

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