[patch 2/2] KVM: LAPIC: cap __delay at lapic_timer_advance_ns

Marcelo Tosatti
2016-06-21T01:29:54Z

[-- Attachment #1: lapic-cap-wait --]
[-- Type: text/plain, Size: 1009 bytes --]

The host timer which emulates the guest LAPIC TSC deadline
timer has its expiration diminished by lapic_timer_advance_ns
nanoseconds. Therefore if, at wait_lapic_expire, a difference
larger than lapic_timer_advance_ns is encountered, delay at most
lapic_timer_advance_ns.

This fixes a problem where the guest can cause the host
to delay for large amounts of time.

Reported-by: Alan Jenkins <alan.christopher.jenkins@gmail.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Index: kvm/arch/x86/kvm/lapic.c
===================================================================
--- kvm.orig/arch/x86/kvm/lapic.c
+++ kvm/arch/x86/kvm/lapic.c
@@ -1164,7 +1164,8 @@ void wait_lapic_expire(struct kvm_vcpu *

/* __delay is delay_tsc whenever the hardware has TSC, thus always. */
if (guest_tsc < tsc_deadline)
- __delay(tsc_deadline - guest_tsc);
+ __delay(max(tsc_deadline - guest_tsc,
+ nsec_to_cycles(vcpu, lapic_timer_advance_ns)));
}

static void start_apic_timer(struct kvm_lapic *apic)