> On 16 Apr 2019, at 20:48, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> 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@xxxxxxxxxx>
> Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Reviewed-by: Liran Alon <liran.alon@xxxxxxxxxx>
> ---
> 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
>
