[PATCH v4 0/5] KVM: LAPIC: Implement Exitless Timer

[Wanpeng Li]

Dedicated instances are currently disturbed by unnecessary jitter due 
to the emulated lapic timers fire on the same pCPUs which vCPUs resident.
There is no hardware virtual timer on Intel for guest like ARM. Both 
programming timer in guest and the emulated timer fires incur vmexits.
This patchset tries to avoid vmexit which is incurred by the emulated 
timer fires in dedicated instance scenario. 

When nohz_full is enabled in dedicated instances scenario, the unpinned 
timer will be moved to the nearest busy housekeepers after commit
9642d18eee2cd (nohz: Affine unpinned timers to housekeepers) and commit 
444969223c8 ("sched/nohz: Fix affine unpinned timers mess"). However, 
KVM always makes lapic timer pinned to the pCPU which vCPU residents, the 
reason is explained by commit 61abdbe0 (kvm: x86: make lapic hrtimer 
pinned). Actually, these emulated timers can be offload to the housekeeping 
cpus since APICv is really common in recent years. The guest timer interrupt 
is injected by posted-interrupt which is delivered by housekeeping cpu 
once the emulated timer fires. 

The host admin should fine tuned, e.g. dedicated instances scenario w/ 
nohz_full cover the pCPUs which vCPUs resident, several pCPUs surplus 
for busy housekeeping, disable mwait/hlt/pause vmexits to keep in non-root  
mode, ~3% redis performance benefit can be observed on Skylake server.

w/o patchset:

            VM-EXIT  Samples  Samples%  Time%   Min Time  Max Time   Avg time

EXTERNAL_INTERRUPT    42916    49.43%   39.30%   0.47us   106.09us   0.71us ( +-   1.09% )

w/ patchset:

            VM-EXIT  Samples  Samples%  Time%   Min Time  Max Time         Avg time

EXTERNAL_INTERRUPT    6871     9.29%     2.96%   0.44us    57.88us   0.72us ( +-   4.02% )

Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>

v3 -> v4:
 * drop the HRTIMER_MODE_ABS_PINNED, add kick after set pending timer
 * don't posted inject already-expired timer

v2 -> v3:
 * disarming the vmx preemption timer when posted_interrupt_inject_timer_enabled()
 * check kvm_hlt_in_guest instead

v1 -> v2:
 * check vcpu_halt_in_guest
 * move module parameter from kvm-intel to kvm
 * add housekeeping_enabled
 * rename apic_timer_expired_pi to kvm_apic_inject_pending_timer_irqs

Wanpeng Li (5):
  KVM: LAPIC: Make lapic timer unpinned
  KVM: LAPIC: inject lapic timer interrupt by posted interrupt
  KVM: LAPIC: Ignore timer migration when lapic timer is injected by pi
  KVM: LAPIC: Don't posted inject already-expired timer
  KVM: LAPIC: add advance timer support to pi_inject_timer

 arch/x86/kvm/lapic.c            | 62 ++++++++++++++++++++++++++++-------------
 arch/x86/kvm/lapic.h            |  3 +-
 arch/x86/kvm/svm.c              |  2 +-
 arch/x86/kvm/vmx/vmx.c          |  5 ++--
 arch/x86/kvm/x86.c              | 11 ++++----
 arch/x86/kvm/x86.h              |  2 ++
 include/linux/sched/isolation.h |  2 ++
 kernel/sched/isolation.c        |  6 ++++
 8 files changed, 64 insertions(+), 29 deletions(-)

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2.7.4