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Subject[PATCH 4.18 033/123] KVM: arm/arm64: Fix potential loss of ptimer interrupts
4.18-stable review patch.  If anyone has any objections, please let me know.


From: Christoffer Dall <>

commit 7afc4ddbf299a13aaf28406783d141a34c6b4f5a upstream.

kvm_timer_update_state() is called when changing the phys timer
configuration registers, either via vcpu reset, as a result of a trap
from the guest, or when userspace programs the registers.

phys_timer_emulate() is in turn called by kvm_timer_update_state() to
either cancel an existing software timer, or program a new software
timer, to emulate the behavior of a real phys timer, based on the change
in configuration registers.

Unfortunately, the interaction between these two functions left a small
race; if the conceptual emulated phys timer should actually fire, but
the soft timer hasn't executed its callback yet, we cancel the timer in
phys_timer_emulate without injecting an irq. This only happens if the
check in kvm_timer_update_state is called before the timer should fire,
which is relatively unlikely, but possible.

The solution is to update the state of the phys timer after calling
phys_timer_emulate, which will pick up the pending timer state and
update the interrupt value.

Note that this leaves the opportunity of raising the interrupt twice,
once in the just-programmed soft timer, and once in
kvm_timer_update_state. Since this always happens synchronously with
the VCPU execution, there is no harm in this, and the guest ever only
sees a single timer interrupt.

Cc: Stable <> # 4.15+
Signed-off-by: Christoffer Dall <>
Signed-off-by: Marc Zyngier <>
Signed-off-by: Greg Kroah-Hartman <>

virt/kvm/arm/arch_timer.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

--- a/virt/kvm/arm/arch_timer.c
+++ b/virt/kvm/arm/arch_timer.c
@@ -295,9 +295,9 @@ static void phys_timer_emulate(struct kv
struct arch_timer_context *ptimer = vcpu_ptimer(vcpu);

- * If the timer can fire now we have just raised the IRQ line and we
- * don't need to have a soft timer scheduled for the future. If the
- * timer cannot fire at all, then we also don't need a soft timer.
+ * If the timer can fire now, we don't need to have a soft timer
+ * scheduled for the future. If the timer cannot fire at all,
+ * then we also don't need a soft timer.
if (kvm_timer_should_fire(ptimer) || !kvm_timer_irq_can_fire(ptimer)) {
soft_timer_cancel(&timer->phys_timer, NULL);
@@ -332,10 +332,10 @@ static void kvm_timer_update_state(struc
level = kvm_timer_should_fire(vtimer);
kvm_timer_update_irq(vcpu, level, vtimer);

+ phys_timer_emulate(vcpu);
if (kvm_timer_should_fire(ptimer) != ptimer->irq.level)
kvm_timer_update_irq(vcpu, !ptimer->irq.level, ptimer);
- phys_timer_emulate(vcpu);

static void vtimer_save_state(struct kvm_vcpu *vcpu)

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