> On 16 Apr 2019, at 14:02, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 12/04/19 22:18, Sean Christopherson 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. Sidestep the headache
>> of shifting time domains by delaying 1ns at a time and breaking the loop
>> when the guest's desired timer delay has been satisfied. Because the
>> advancement, which caps the delay to avoid unbounded busy waiting, is
>> stored in nanoseconds, the current advancement time can simply be used
>> as a loop terminator since we're delaying 1ns at a time (plus the few
>> cycles of overhead for running the code).
>
> A 1 nanosecond advance (3-5 clock cycles) is even shorter than the time
> taken to execute kvm_read_l1_tsc. I would just replace it with
> cpu_relax(); I can do the change when applying.
>
> Paolo
Paolo, there is also another approach Sean and I were discussing.
I think it is more elegant.
See previous comments in this thread.
-Liran
>
>> Cc: Liran Alon <liran.alon@xxxxxxxxxx>
>> Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>> ---
>> arch/x86/kvm/lapic.c | 13 +++++++------
>> 1 file changed, 7 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>> index 92446cba9b24..e797e3145a8b 100644
>> --- a/arch/x86/kvm/lapic.c
>> +++ b/arch/x86/kvm/lapic.c
>> @@ -1486,7 +1486,8 @@ static bool lapic_timer_int_injected(struct kvm_vcpu *vcpu)
>> void wait_lapic_expire(struct kvm_vcpu *vcpu)
>> {
>> struct kvm_lapic *apic = vcpu->arch.apic;
>> - u64 guest_tsc, tsc_deadline, ns;
>> + u32 timer_advance_ns = lapic_timer_advance_ns;
>> + u64 guest_tsc, tmp_tsc, tsc_deadline, ns;
>>
>> if (!lapic_in_kernel(vcpu))
>> return;
>> @@ -1499,13 +1500,13 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu)
>>
>> tsc_deadline = apic->lapic_timer.expired_tscdeadline;
>> apic->lapic_timer.expired_tscdeadline = 0;
>> - guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
>> + tmp_tsc = 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)));
>> + for (ns = 0; tmp_tsc < tsc_deadline && ns < timer_advance_ns; ns++) {
>> + ndelay(1);
>> + tmp_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
>> + }
>>
>> if (!lapic_timer_advance_adjust_done) {
>> /* too early */
>>
>
