[PATCH v7 2/7] KVM: arm64: Separate guest/host counter offset values

[Oliver Upton]

In some instances, a VMM may want to update the guest's counter-timer
offset in a transparent manner, meaning that changes to the hardware
value do not affect the synthetic register presented to the guest or the
VMM through said guest's architectural state. Lay the groundwork to
separate guest offset register writes from the hardware values utilized
by KVM.

Signed-off-by: Oliver Upton <oupton at google.com>
Reviewed-by: Andrew Jones <drjones at redhat.com>
---
 arch/arm64/kvm/arch_timer.c  | 42 +++++++++++++++++++++++++++---------
 include/kvm/arm_arch_timer.h |  3 +++
 2 files changed, 35 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
index c0101db75ad4..cf2f4a034dbe 100644
--- a/arch/arm64/kvm/arch_timer.c
+++ b/arch/arm64/kvm/arch_timer.c
@@ -84,11 +84,9 @@ u64 timer_get_cval(struct arch_timer_context *ctxt)
 
 static u64 timer_get_offset(struct arch_timer_context *ctxt)
 {
-	struct kvm_vcpu *vcpu = ctxt->vcpu;
-
 	switch(arch_timer_ctx_index(ctxt)) {
 	case TIMER_VTIMER:
-		return __vcpu_sys_reg(vcpu, CNTVOFF_EL2);
+		return ctxt->host_offset;
 	default:
 		return 0;
 	}
@@ -128,17 +126,33 @@ static void timer_set_cval(struct arch_timer_context *ctxt, u64 cval)
 
 static void timer_set_offset(struct arch_timer_context *ctxt, u64 offset)
 {
-	struct kvm_vcpu *vcpu = ctxt->vcpu;
-
 	switch(arch_timer_ctx_index(ctxt)) {
 	case TIMER_VTIMER:
-		__vcpu_sys_reg(vcpu, CNTVOFF_EL2) = offset;
+		ctxt->host_offset = offset;
 		break;
 	default:
 		WARN(offset, "timer %ld\n", arch_timer_ctx_index(ctxt));
 	}
 }
 
+static void timer_set_guest_offset(struct arch_timer_context *ctxt, u64 offset)
+{
+	struct kvm_vcpu *vcpu = ctxt->vcpu;
+
+	switch (arch_timer_ctx_index(ctxt)) {
+	case TIMER_VTIMER: {
+		u64 host_offset = timer_get_offset(ctxt);
+
+		host_offset += offset - __vcpu_sys_reg(vcpu, CNTVOFF_EL2);
+		__vcpu_sys_reg(vcpu, CNTVOFF_EL2) = offset;
+		timer_set_offset(ctxt, host_offset);
+		break;
+	}
+	default:
+		WARN_ONCE(offset, "timer %ld\n", arch_timer_ctx_index(ctxt));
+	}
+}
+
 u64 kvm_phys_timer_read(void)
 {
 	return timecounter->cc->read(timecounter->cc);
@@ -749,7 +763,8 @@ int kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu)
 
 /* Make offset updates for all timer contexts atomic */
 static void update_timer_offset(struct kvm_vcpu *vcpu,
-				enum kvm_arch_timers timer, u64 offset)
+				enum kvm_arch_timers timer, u64 offset,
+				bool guest_visible)
 {
 	int i;
 	struct kvm *kvm = vcpu->kvm;
@@ -758,13 +773,20 @@ static void update_timer_offset(struct kvm_vcpu *vcpu,
 	lockdep_assert_held(&kvm->lock);
 
 	kvm_for_each_vcpu(i, tmp, kvm)
-		timer_set_offset(vcpu_get_timer(tmp, timer), offset);
+		if (guest_visible)
+			timer_set_guest_offset(vcpu_get_timer(tmp, timer),
+					       offset);
+		else
+			timer_set_offset(vcpu_get_timer(tmp, timer), offset);
 
 	/*
 	 * When called from the vcpu create path, the CPU being created is not
 	 * included in the loop above, so we just set it here as well.
 	 */
-	timer_set_offset(vcpu_get_timer(vcpu, timer), offset);
+	if (guest_visible)
+		timer_set_guest_offset(vcpu_get_timer(vcpu, timer), offset);
+	else
+		timer_set_offset(vcpu_get_timer(vcpu, timer), offset);
 }
 
 static void update_vtimer_cntvoff(struct kvm_vcpu *vcpu, u64 cntvoff)
@@ -772,7 +794,7 @@ static void update_vtimer_cntvoff(struct kvm_vcpu *vcpu, u64 cntvoff)
 	struct kvm *kvm = vcpu->kvm;
 
 	mutex_lock(&kvm->lock);
-	update_timer_offset(vcpu, TIMER_VTIMER, cntvoff);
+	update_timer_offset(vcpu, TIMER_VTIMER, cntvoff, true);
 	mutex_unlock(&kvm->lock);
 }
 
diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
index 51c19381108c..9d65d4a29f81 100644
--- a/include/kvm/arm_arch_timer.h
+++ b/include/kvm/arm_arch_timer.h
@@ -42,6 +42,9 @@ struct arch_timer_context {
 	/* Duplicated state from arch_timer.c for convenience */
 	u32				host_timer_irq;
 	u32				host_timer_irq_flags;
+
+	/* offset relative to the host's physical counter-timer */
+	u64				host_offset;
 };
 
 struct timer_map {
-- 
2.33.0.rc1.237.g0d66db33f3-goog