On Mon, Oct 12, 2020 at 11:31 AM Nadav Amit <nadav.amit@xxxxxxxxx> wrote: > > > On Oct 12, 2020, at 11:29 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > > > On 12/10/20 20:17, Nadav Amit wrote: > >>> KVM clearly doesn't adhere to the architectural specification. I don't > >>> know what is wrong with your Broadwell machine. > >> Are you saying that the test is expected to fail on KVM? And that Sean’s > >> failures are expected? > > > > It's not expected to fail, but it's apparently broken. > > Hm… Based on my results on bare-metal, it might be an architectural issue or > a test issue, and not a KVM issue. >From section 25.5.1 of the SDM, volume 3: If the last VM entry was performed with the 1-setting of “activate VMX-preemption timer” VM-execution control, the VMX-preemption timer counts down (from the value loaded by VM entry; see Section 26.7.4) in VMX non- root operation. When the timer counts down to zero, it stops counting down and a VM exit occurs (see Section 25.2). The test is actually quite lax, in that it doesn't start tracking VMX non-root operation time until actually in the guest. Hardware is free to start tracking VMX non-root operation time during VM-entry. If the test can both observe a TSC value after the VMX-preemption timer deadline *and* store that value to memory, then the store instruction must have started executing after the VMX-preemption timer has counted down to zero. Per the SDM, a VM-exit should have occurred before the store could retire. Of course, there could be a test bug.
