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

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Date:   Tue, 15 Oct 2019 12:49:52 -0400
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH 12/14] KVM: retpolines: x86: eliminate retpoline from
 vmx.c exit handlers

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
> EXIT_REASON_PAUSE_INSTRUCTION, EXIT_REASON_PENDING_INTERRUPT,
> EXIT_REASON_EXTERNAL_INTERRUPT, EXIT_REASON_HLT, EXIT_REASON_MSR_READ,
> EXIT_REASON_CPUID.
> 
> 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.

I'm pretty sure HLT/EXTERNAL_INTERRUPT/PENDING_INTERRUPT should be
included.

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.