[patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe

Marcelo Tosatti
2014-11-25T17:21:10Z

[-- Attachment #1: kvm-lapic-irqsafe --]
[-- Type: text/plain, Size: 2679 bytes --]

Since lapic timer handler only wakes up a simple waitqueue,
it can be executed from hardirq context.

Reduces average cyclictest latency by 3us.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

---
arch/x86/kvm/lapic.c | 42 +++++++++++++++++++++++++++++++++++++++---
1 file changed, 39 insertions(+), 3 deletions(-)

Index: linux-stable-rt/arch/x86/kvm/lapic.c
===================================================================
--- linux-stable-rt.orig/arch/x86/kvm/lapic.c 2014-11-25 14:14:38.636810068 -0200
+++ linux-stable-rt/arch/x86/kvm/lapic.c 2014-11-25 14:17:28.850567059 -0200
@@ -1031,8 +1031,38 @@
apic->divide_count);
}

+
+static enum hrtimer_restart apic_timer_fn(struct hrtimer *data);
+
+static void apic_timer_expired(struct hrtimer *data)
+{
+ int ret, i = 0;
+ enum hrtimer_restart r;
+ struct kvm_timer *ktimer = container_of(data, struct kvm_timer, timer);
+
+ r = apic_timer_fn(data);
+
+ if (r == HRTIMER_RESTART) {
+ do {
+ ret = hrtimer_start_expires(data, HRTIMER_MODE_ABS);
+ if (ret == -ETIME)
+ hrtimer_add_expires_ns(&ktimer->timer,
+ ktimer->period);
+ i++;
+ } while (ret == -ETIME && i < 10);
+
+ if (ret == -ETIME) {
+ printk(KERN_ERR "%s: failed to reprogram timer\n",
+ __func__);
+ WARN_ON(1);
+ }
+ }
+}
+
+
static void start_apic_timer(struct kvm_lapic *apic)
{
+ int ret;
ktime_t now;
atomic_set(&apic->lapic_timer.pending, 0);

@@ -1062,9 +1092,11 @@
}
}

- hrtimer_start(&apic->lapic_timer.timer,
+ ret = hrtimer_start(&apic->lapic_timer.timer,
ktime_add_ns(now, apic->lapic_timer.period),
HRTIMER_MODE_ABS);
+ if (ret == -ETIME)
+ apic_timer_expired(&apic->lapic_timer.timer);

apic_debug("%s: bus cycle is %" PRId64 "ns, now 0x%016"
PRIx64 ", "
@@ -1094,8 +1126,10 @@
ns = (tscdeadline - guest_tsc) * 1000000ULL;
do_div(ns, this_tsc_khz);
}
- hrtimer_start(&apic->lapic_timer.timer,
+ ret = hrtimer_start(&apic->lapic_timer.timer,
ktime_add_ns(now, ns), HRTIMER_MODE_ABS);
+ if (ret == -ETIME)
+ apic_timer_expired(&apic->lapic_timer.timer);

local_irq_restore(flags);
}
@@ -1581,6 +1615,7 @@
hrtimer_init(&apic->lapic_timer.timer, CLOCK_MONOTONIC,
HRTIMER_MODE_ABS);
apic->lapic_timer.timer.function = apic_timer_fn;
+ apic->lapic_timer.timer.irqsafe = 1;

/*
* APIC is created enabled. This will prevent kvm_lapic_set_base from
@@ -1699,7 +1734,8 @@

timer = &vcpu->arch.apic->lapic_timer.timer;
if (hrtimer_cancel(timer))
- hrtimer_start_expires(timer, HRTIMER_MODE_ABS);
+ if (hrtimer_start_expires(timer, HRTIMER_MODE_ABS) == -ETIME)
+ apic_timer_expired(timer);
}

/*