linux-kernel - [PATCH 0/5] KVM: x86: cleanup and minimal speedup for APICv

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Date:   Fri, 14 Oct 2016 20:21:26 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     rkrcmar@...hat.com, yang.zhang.wz@...il.com, feng.wu@...el.com,
        mst@...hat.com
Subject: [PATCH 0/5] KVM: x86: cleanup and minimal speedup for APICv

On some benchmarks (e.g. netperf with ioeventfd disabled), APICv
posted interrupts turn out to be slower than interrupt injection via
KVM_REQ_EVENT.

These patches save 40 cycles on kvm-unit-tests "inl" tests (1-2%), but I
suspect the effect is bigger on workloads that inject interrupts heavily.

Patches 1-2 are micro-optimization of the APICv vmentry code.

The interesting one is patch 3.  The aim is for APICv to not use
KVM_REQ_EVENT at all for interrupts, therefore turning APICv's weakness
(having to look at PIR on every vmentry) into a strength (because
checking PIR.ON is cheaper than processing KVM_REQ_EVENT).  I find that
the new code overall makes more sense than the old one, and I like that
more of the logic is contained within vmx_deliver_posted_interrupt and
vmx_sync_pir_to_irr.  KVM_REQ_EVENT did protect from mistakes, so the new
code is more delicate, but I still prefer that to duct tape. :)

Patches 4 and 5 are other cleanups.

Paolo

Paolo Bonzini (5):
  KVM: x86: avoid atomic operations on APICv vmentry
  KVM: x86: do not scan IRR twice on APICv vmentry
  KVM: x86: do not use KVM_REQ_EVENT for APICv interrupt injection
  KVM: x86: remove unnecessary sync_pir_to_irr
  KVM: vmx: clear pending interrupts on KVM_SET_LAPIC

 arch/x86/kvm/lapic.c | 45 +++++++++++++++++++++++++++++----------------
 arch/x86/kvm/lapic.h |  4 ++--
 arch/x86/kvm/vmx.c   | 44 ++++++++++++++++++++++++++++++++------------
 arch/x86/kvm/x86.c   | 45 +++++++++++++++++++++++++--------------------
 4 files changed, 88 insertions(+), 50 deletions(-)

-- 
1.8.3.1