Re: [PATCH 12/14] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers

From: Andrea Arcangeli
Date: Tue Oct 15 2019 - 12:49:56 EST

On Tue, Oct 15, 2019 at 10:28:39AM +0200, Paolo Bonzini wrote:
> If you're including EXIT_REASON_EPT_MISCONFIG (MMIO access) then you
> should include EXIT_REASON_IO_INSTRUCTION too. Depending on the devices
> that are in the guest, the doorbell register might be MMIO or PIO.

The fact outb/inb devices exists isn't the question here. The question
you should clarify is: which of the PIO devices is performance
critical as much as MMIO with virtio/vhost? I mean even on real
hardware those devices aren't performance critical. I didn't run into
PIO drivers with properly configured guests.

> So, the difference between my suggested list (which I admit is just
> based on conjecture, not benchmarking) is that you add
> Which of these make a difference for the hrtimer testcase? It's of
> course totally fine to use benchmarks to prove that my intuition was
> bad---but you must also use them to show why your intuition is right. :)

The hrtimer flood hits on this:

MSR_WRITE 338793 56.54% 5.51% 0.33us 34.44us 0.44us ( +- 0.20% )
PENDING_INTERRUPT 168431 28.11% 2.52% 0.36us 32.06us 0.40us ( +- 0.28% )
PREEMPTION_TIMER 91723 15.31% 1.32% 0.34us 30.51us 0.39us ( +- 0.41% )
EXTERNAL_INTERRUPT 234 0.04% 0.00% 0.25us 5.53us 0.43us ( +- 5.67% )
HLT 65 0.01% 90.64% 0.49us 319933.79us 37562.71us ( +- 21.68% )
MSR_READ 6 0.00% 0.00% 0.67us 1.96us 1.06us ( +- 17.97% )
EPT_MISCONFIG 6 0.00% 0.01% 3.09us 105.50us 26.76us ( +- 62.10% )

PENDING_INTERRUPT is the big missing thing in your list. It probably
accounts for the bulk of slowdown from your list. However I could
imagine other loads with higher external interrupt/hlt/rdmsr than the
hrtimer one so I didn't drop those. Other loads are hitting on a flood
of HLT and from host standpoint it's no a slow path. Not all OS have
the cpuidle haltpoll governor to mitigate the HLT frequency.


The least useful are PAUSE, CPUID and MSR_READ, we could considering
dropping some of those (in the short term cpuid helps for benchmarking
to more accurately measure the performance improvement of not hitting
the retpoline there). I simply could imagine some load hitting
frequently on those too so I didn't drop them.

I also wonder if VMCALL should be added, certain loads hit on fairly
frequent VMCALL, but none of the one I benchmarked.