On Tue, Jun 11, 2019 at 09:38:18AM +0800, Wanpeng Li wrote: > On Tue, 11 Jun 2019 at 09:21, Sean Christopherson > <sean.j.christopherson@xxxxxxxxx> wrote: > > > > On Fri, May 31, 2019 at 02:40:13PM +0800, Wanpeng Li wrote: > > > From: Wanpeng Li <wanpengli@xxxxxxxxxxx> > > > > > > Advance lapic timer tries to hidden the hypervisor overhead between the > > > host emulated timer fires and the guest awares the timer is fired. However, > > > even though after more sustaining optimizations, kvm-unit-tests/tscdeadline_latency > > > still awares ~1000 cycles latency since we lost the time between the end of > > > wait_lapic_expire and the guest awares the timer is fired. There are > > > codes between the end of wait_lapic_expire and the world switch, furthermore, > > > the world switch itself also has overhead. Actually the guest_tsc is equal > > > to the target deadline time in wait_lapic_expire is too late, guest will > > > aware the latency between the end of wait_lapic_expire() and after vmentry > > > to the guest. This patch takes this time into consideration. > > > > > > The vmentry_lapic_timer_advance_ns module parameter should be well tuned by > > > host admin, setting bit 0 to 1 to finally cache parameter in KVM. This patch > > > can reduce average cyclictest latency from 3us to 2us on Skylake server. > > > (guest w/ nohz=off, idle=poll, host w/ preemption_timer=N, the cyclictest > > > latency is not too sensitive when preemption_timer=Y for this optimization in > > > my testing), kvm-unit-tests/tscdeadline_latency can reach 0. > > > > > > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> > > > Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> > > > Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> > > > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx> > > > --- > > > NOTE: rebase on https://lkml.org/lkml/2019/5/20/449 > > > v1 -> v2: > > > * rename get_vmentry_advance_delta to get_vmentry_advance_cycles > > > * cache vmentry_advance_cycles by setting param bit 0 > > > * add param max limit > > > > > > arch/x86/kvm/lapic.c | 38 +++++++++++++++++++++++++++++++++++--- > > > arch/x86/kvm/lapic.h | 3 +++ > > > arch/x86/kvm/vmx/vmx.c | 2 +- > > > arch/x86/kvm/x86.c | 9 +++++++++ > > > arch/x86/kvm/x86.h | 2 ++ > > > 5 files changed, 50 insertions(+), 4 deletions(-) > > > > > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > > > index fcf42a3..60587b5 100644 > > > --- a/arch/x86/kvm/lapic.c > > > +++ b/arch/x86/kvm/lapic.c > > > @@ -1531,6 +1531,38 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu, > > > apic->lapic_timer.timer_advance_ns = timer_advance_ns; > > > } > > > > > > +#define MAX_VMENTRY_ADVANCE_NS 1000 > > > + > > > +u64 compute_vmentry_advance_cycles(struct kvm_vcpu *vcpu) > > > > This can be static, unless get_vmentry_advance_cycles() is moved to > > lapic.h, in which case compute_vmentry_advance_cycles() would need to be > > exported. > > Thanks for the review, Sean. I think Paolo has already drop this one. > https://lkml.org/lkml/2019/5/31/210 I couldn't tell if Paolo's response was "no, don't do that" or "let's be careful". :-)
