[PATCH v3 2/9] KVM: lapic: Convert guest TSC to host time domain when delaying

[Sean Christopherson <sean.j.christopherson@intel.com>]

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>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.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