[PATCH v6 11/12] ARM: KVM: Guest wait-for-interrupts (WFI) support

Christoffer Dall
2012-02-23T07:33:23Z

When the guest executes a WFI instruction the operation is trapped to
KVM, which emulates the instruction in software. There is no correlation
between a guest executing a WFI instruction and actually putting the
hardware into a low-power mode, since a KVM guest is essentially a
process and the WFI instruction can be seen as 'sleep' call from this
process. Therefore, we flag the VCPU to be in wait_for_interrupts mode
and call the main KVM function kvm_vcpu_block() function. This function
will put the thread on a wait-queue and call schedule.

When an interrupt comes in through KVM_IRQ_LINE (see previous patch) we
signal the VCPU thread and unflag the VCPU to no longer wait for
interrupts. All calls to kvm_arch_vcpu_ioctl_run() result in a call to
kvm_vcpu_block() as long as the VCPU is in wfi-mode.

Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>
---
arch/arm/kvm/arm.c | 15 ++++++++++++++-
arch/arm/kvm/emulate.c | 12 ++++++++++++
arch/arm/kvm/trace.h | 16 ++++++++++++++++
3 files changed, 42 insertions(+), 1 deletions(-)

diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
index 26fee60..3772847 100644
--- a/arch/arm/kvm/arm.c
+++ b/arch/arm/kvm/arm.c
@@ -268,9 +268,17 @@ int kvm_arch_vcpu_ioctl_set_mpstate(struct kvm_vcpu *vcpu,
return -EINVAL;
}

+/**
+ * kvm_arch_vcpu_runnable - determine if the vcpu can be scheduled
+ * @v: The VCPU pointer
+ *
+ * If the guest CPU is not waiting for interrupts (or waiting and
+ * an interrupt is pending) then it is by definition runnable.
+ */
int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
{
- return 0;
+ return !!v->arch.irq_lines ||
+ !v->arch.wait_for_interrupts;
}

int kvm_arch_vcpu_in_guest_mode(struct kvm_vcpu *v)
@@ -448,6 +456,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
break;
}

+ if (vcpu->arch.wait_for_interrupts)
+ kvm_vcpu_block(vcpu);
+
/*
* Enter the guest
*/
@@ -520,6 +531,8 @@ static int kvm_arch_vm_ioctl_irq_line(struct kvm *kvm,
* trigger a world-switch round on the running physical CPU to set the
* virtual IRQ/FIQ fields in the HCR appropriately.
*/
+ if (irq_level->level)
+ vcpu->arch.wait_for_interrupts = 0;
kvm_vcpu_kick(vcpu);

return 0;
diff --git a/arch/arm/kvm/emulate.c b/arch/arm/kvm/emulate.c
index 77ccc2b..20269f8 100644
--- a/arch/arm/kvm/emulate.c
+++ b/arch/arm/kvm/emulate.c
@@ -335,8 +335,20 @@ unsupp_err_out:
return -EINVAL;
}

+/**
+ * kvm_handle_wfi - handle a wait-for-interrupts instruction executed by a guest
+ * @vcpu: the vcpu pointer
+ * @run: the kvm_run structure pointer
+ *
+ * Simply sets the wait_for_interrupts flag on the vcpu structure, which will
+ * halt execution of world-switches and schedule other host processes until
+ * there is an incoming IRQ or FIQ to the VM.
+ */
int kvm_handle_wfi(struct kvm_vcpu *vcpu, struct kvm_run *run)
{
+ trace_kvm_wfi(vcpu->arch.regs.pc);
+ if (!vcpu->arch.irq_lines)
+ vcpu->arch.wait_for_interrupts = 1;
return 0;
}

diff --git a/arch/arm/kvm/trace.h b/arch/arm/kvm/trace.h
index bd3a6cc..fc68394 100644
--- a/arch/arm/kvm/trace.h
+++ b/arch/arm/kvm/trace.h
@@ -90,6 +90,22 @@ TRACE_EVENT(kvm_emulate_cp15_imp,
__entry->CRm, __entry->Op2)
);

+TRACE_EVENT(kvm_wfi,
+ TP_PROTO(unsigned long vcpu_pc),
+ TP_ARGS(vcpu_pc),
+
+ TP_STRUCT__entry(
+ __field( unsigned long, vcpu_pc )
+ ),
+
+ TP_fast_assign(
+ __entry->vcpu_pc = vcpu_pc;
+ ),
+
+ TP_printk("guest executed wfi at: 0x%08lx", __entry->vcpu_pc)
+);
+
+
#endif /* _TRACE_KVM_H */

#undef TRACE_INCLUDE_PATH