| Message ID | 20190416174857.506-3-sean.j.christopherson@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: lapic: Fix a variety of timer adv issues | expand |
> On 16 Apr 2019, at 20:48, Sean Christopherson <sean.j.christopherson@intel.com> wrote: > > To minimize the latency of timer interrupts as observed by the guest, > KVM adjusts the values it programs into the host timers to account for > the host's overhead of programming and handling the timer event. In > the event that the adjustments are too aggressive, i.e. the timer fires > earlier than the guest expects, KVM busy waits immediately prior to > entering the guest. > > Currently, KVM manually converts the delay from nanoseconds to clock > cycles. But, the conversion is done in the guest's time domain, while > the delay occurs in the host's time domain, i.e. the delay may not be > accurate and could wait too little or too long. > > Convert the delay from guest clock cycles to host nanoseconds and use > ndelay() instead of __delay() to provide more accurate timing. This > also avoids the need to convert lapic_timer_advance_ns, which is used > to cap the delay, to guest clock cycles. > > Cc: Liran Alon <liran.alon@oracle.com> > Cc: Wanpeng Li <wanpengli@tencent.com> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> Reviewed-by: Liran Alon <liran.alon@oracle.com> > --- > arch/x86/kvm/lapic.c | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > index 92446cba9b24..5891c0badfa6 100644 > --- a/arch/x86/kvm/lapic.c > +++ b/arch/x86/kvm/lapic.c > @@ -1502,10 +1502,12 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu) > guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()); > trace_kvm_wait_lapic_expire(vcpu->vcpu_id, guest_tsc - tsc_deadline); > > - /* __delay is delay_tsc whenever the hardware has TSC, thus always. */ > - if (guest_tsc < tsc_deadline) > - __delay(min(tsc_deadline - guest_tsc, > - nsec_to_cycles(vcpu, lapic_timer_advance_ns))); > + /* ndelay uses delay_tsc whenever the hardware has TSC, thus always. */ > + if (guest_tsc < tsc_deadline) { > + ns = (tsc_deadline - guest_tsc) * 1000000ULL; > + do_div(ns, vcpu->arch.virtual_tsc_khz); > + ndelay(min_t(u32, ns, lapic_timer_advance_ns)); > + } > > if (!lapic_timer_advance_adjust_done) { > /* too early */ > -- > 2.21.0 >
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index 92446cba9b24..5891c0badfa6 100644 --- a/arch/x86/kvm/lapic.c +++ b/arch/x86/kvm/lapic.c @@ -1502,10 +1502,12 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu) guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()); trace_kvm_wait_lapic_expire(vcpu->vcpu_id, guest_tsc - tsc_deadline); - /* __delay is delay_tsc whenever the hardware has TSC, thus always. */ - if (guest_tsc < tsc_deadline) - __delay(min(tsc_deadline - guest_tsc, - nsec_to_cycles(vcpu, lapic_timer_advance_ns))); + /* ndelay uses delay_tsc whenever the hardware has TSC, thus always. */ + if (guest_tsc < tsc_deadline) { + ns = (tsc_deadline - guest_tsc) * 1000000ULL; + do_div(ns, vcpu->arch.virtual_tsc_khz); + ndelay(min_t(u32, ns, lapic_timer_advance_ns)); + } if (!lapic_timer_advance_adjust_done) { /* too early */
To minimize the latency of timer interrupts as observed by the guest, KVM adjusts the values it programs into the host timers to account for the host's overhead of programming and handling the timer event. In the event that the adjustments are too aggressive, i.e. the timer fires earlier than the guest expects, KVM busy waits immediately prior to entering the guest. Currently, KVM manually converts the delay from nanoseconds to clock cycles. But, the conversion is done in the guest's time domain, while the delay occurs in the host's time domain, i.e. the delay may not be accurate and could wait too little or too long. Convert the delay from guest clock cycles to host nanoseconds and use ndelay() instead of __delay() to provide more accurate timing. This also avoids the need to convert lapic_timer_advance_ns, which is used to cap the delay, to guest clock cycles. Cc: Liran Alon <liran.alon@oracle.com> Cc: Wanpeng Li <wanpengli@tencent.com> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> --- arch/x86/kvm/lapic.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-)