On Mon, 2018-07-16 at 08:40 -0700, Paul E. McKenney wrote:
> Most of the weekend was devoted to testing today's upcoming pull request,
> but I did get a bit more testing done on this.
> 
> I was able to make this happen more often by tweaking rcutorture a
> bit, but I still do not yet have statistically significant results.
> Nevertheless, I have thus far only seen failures with David's patch or
> with both David's and my patch.  And I actually got a full-up rcutorture
> failure (a too-short grace period) in addition to the aforementioned
> close calls.
> 
> Over this coming week I expect to devote significant testing time to
> the commit just prior to David's in my stack.  If I don't see failures
> on that commit, we will need to spent some quality time with the KVM
> folks on whether or not kvm_x86_ops->run() and friends have the option of
> failing to return, but instead causing control to pop up somewhere else.
> Or someone could tell me how I am being blind to some obvious bug in
> the two commits that allow RCU to treat KVM guest-OS execution as an
> extended quiescent state.  ;-)

One thing we can try, if my patch is implicated, is moving the calls to
rcu_kvm_en{ter,xit} closer to the actual VM entry. Let's try putting
them around the large asm block in arch/x86/kvm/vmx.c::vmx_vcpu_run()
for example. If that fixes it, then we know we've missed something else
interesting that's happening in the middle.

Testing on Skylake shows a guest CPUID goes from ~3000 cycles to ~3500
with this patch, so in the next iteration it definitely needs to be
ifdef CONFIG_NO_HZ_FULL anyway, because it's actually required there
(AFAICT) and it's too expensive otherwise as Christian pointed out.