Re: [kvm-unit-tests PATCH] x86: VMX: Add a VMX-preemption timer expiration test

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Sat, Oct 10, 2020 at 01:42:26AM -0700, Nadav Amit wrote:
> > 
> > On May 8, 2020, at 1:39 PM, Jim Mattson <jmattson@google.com> wrote:
> > 
> > When the VMX-preemption timer is activated, code executing in VMX
> > non-root operation should never be able to record a TSC value beyond
> > the deadline imposed by adding the scaled VMX-preemption timer value
> > to the first TSC value observed by the guest after VM-entry.
> > 
> > Signed-off-by: Jim Mattson <jmattson@google.com>
> > Reviewed-by: Peter Shier <pshier@google.com>
> 
> This test failed on my bare-metal machine (Broadwell):
> 
> Test suite: vmx_preemption_timer_expiry_test
> FAIL: Last stored guest TSC (44435478250637180) < TSC deadline (44435478250419552)
> 
> Any hints why, perhaps based on the motivation for the test?

This test also fails intermittently on my Haswell and Coffee Lake systems when
running on KVM.  I haven't done any "debug" beyond a quick glance at the test.

The intent of the test is to verify that KVM injects preemption timer VM-Exits
without violating the architectural guarantees of the timer, e.g. that the exit
isn't delayed by something else happening in the system.