[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 #8 from Jim Mattson (jmattson@google.com) ---
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?

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