[PATCH 0/3] KVM: LAPIC: Optimize timer latency further

[Wanpeng Li <kernellwp@gmail.com>]

Advance lapic timer tries to hidden the hypervisor overhead between host 
timer fires and the guest awares the timer is fired. However, it just hidden 
the time between apic_timer_fn/handle_preemption_timer -> wait_lapic_expire, 
instead of the real position of vmentry which is mentioned in the orignial 
commit d0659d946be0 ("KVM: x86: add option to advance tscdeadline hrtimer 
expiration"). There is 700+ cpu cycles between the end of wait_lapic_expire 
and before world switch on my haswell desktop, it will be 2400+ cycles if 
vmentry_l1d_flush is tuned to always. 

This patchset tries to narrow the last gap, it measures the time between 
the end of wait_lapic_expire and before world switch, we take this 
time into consideration when busy waiting, otherwise, the guest still 
awares the latency between wait_lapic_expire and world switch, we also 
consider this when adaptively tuning the timer advancement. The patch 
can reduce 50% latency (~1600+ cycles to ~800+ cycles on a haswell 
desktop) for kvm-unit-tests/tscdeadline_latency when testing busy waits.

Wanpeng Li (3):
  KVM: LAPIC: Extract adaptive tune timer advancement logic
  KVM: LAPIC: Fix lapic_timer_advance_ns parameter overflow
  KVM: LAPIC: Optimize timer latency further

 arch/x86/kvm/lapic.c   | 78 ++++++++++++++++++++++++++++++++++----------------
 arch/x86/kvm/lapic.h   |  8 ++++++
 arch/x86/kvm/vmx/vmx.c |  2 ++
 arch/x86/kvm/x86.c     |  2 +-
 4 files changed, 64 insertions(+), 26 deletions(-)

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2.7.4