From: Andrea Arcangeli <aarcange@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Sean Christopherson <sean.j.christopherson@intel.com>
Subject: Re: [PATCH 12/14] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers
Date: Tue, 15 Oct 2019 12:49:52 -0400 [thread overview]
Message-ID: <20191015164952.GE331@redhat.com> (raw)
In-Reply-To: <933ca564-973d-645e-fe9c-9afb64edba5b@redhat.com>
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.
next prev parent reply other threads:[~2019-10-15 16:49 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-28 17:23 [PATCH 00/14] KVM monolithic v2 Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 01/14] KVM: monolithic: x86: remove kvm.ko Andrea Arcangeli
2019-10-15 1:31 ` Sean Christopherson
2019-10-15 3:18 ` Sean Christopherson
2019-10-15 8:32 ` Paolo Bonzini
2019-09-28 17:23 ` [PATCH 02/14] KVM: monolithic: x86: disable linking vmx and svm at the same time into the kernel Andrea Arcangeli
2019-10-15 3:16 ` Sean Christopherson
2019-10-15 8:21 ` Paolo Bonzini
2019-10-15 15:23 ` Sean Christopherson
2019-09-28 17:23 ` [PATCH 04/14] KVM: monolithic: x86: handle the request_immediate_exit variation Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 05/14] KVM: monolithic: add more section prefixes in the KVM common code Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 06/14] KVM: monolithic: x86: remove __exit section prefix from machine_unsetup Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 07/14] KVM: monolithic: x86: remove __init section prefix from kvm_x86_cpu_has_kvm_support Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 08/14] KVM: monolithic: x86: remove exports Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 09/14] KVM: monolithic: remove exports from KVM common code Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 10/14] KVM: monolithic: x86: drop the kvm_pmu_ops structure Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 11/14] KVM: x86: optimize more exit handlers in vmx.c Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 12/14] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers Andrea Arcangeli
2019-10-15 8:28 ` Paolo Bonzini
2019-10-15 16:49 ` Andrea Arcangeli [this message]
2019-10-15 19:46 ` Paolo Bonzini
2019-10-15 20:35 ` Andrea Arcangeli
2019-10-15 22:22 ` Paolo Bonzini
2019-10-15 23:42 ` Andrea Arcangeli
2019-10-16 7:07 ` Paolo Bonzini
2019-10-16 16:50 ` Andrea Arcangeli
2019-10-16 17:01 ` Paolo Bonzini
2019-09-28 17:23 ` [PATCH 13/14] KVM: retpolines: x86: eliminate retpoline from svm.c " Andrea Arcangeli
2019-09-28 17:23 ` [PATCH 14/14] x86: retpolines: eliminate retpoline from msr event handlers Andrea Arcangeli
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