KVM: arm64: nv: timers: Add a per-timer, per-vcpu offset

Being able to set a global offset isn't enough.

With NV, we also need to a per-vcpu, per-timer offset (for example,
CNTVCT_EL0 being offset by CNTVOFF_EL2).

Use a similar method as the VM-wide offset to have a timer point
to the shadow register that contains the offset value.
Reviewed-by: Colton Lewis <coltonlewis@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230330174800.2677007-17-maz@kernel.org

arch/arm64/kvm/arch_timer.c

@@ -89,10 +89,17 @@ u64 timer_get_cval(struct arch_timer_context *ctxt)
 
 static u64 timer_get_offset(struct arch_timer_context *ctxt)
 {
-	if (ctxt && ctxt->offset.vm_offset)
-		return *ctxt->offset.vm_offset;
+	u64 offset = 0;
 
-	return 0;
+	if (!ctxt)
+		return 0;
+
+	if (ctxt->offset.vm_offset)
+		offset += *ctxt->offset.vm_offset;
+	if (ctxt->offset.vcpu_offset)
+		offset += *ctxt->offset.vcpu_offset;
+
+	return offset;
 }
 
 static void timer_set_ctl(struct arch_timer_context *ctxt, u32 ctl)

arch/arm64/kvm/hyp/include/hyp/switch.h

@@ -353,6 +353,8 @@ static bool kvm_hyp_handle_cntpct(struct kvm_vcpu *vcpu)
 
 	if (ctxt->offset.vm_offset)
 		val -= *kern_hyp_va(ctxt->offset.vm_offset);
+	if (ctxt->offset.vcpu_offset)
+		val -= *kern_hyp_va(ctxt->offset.vcpu_offset);
 
 	vcpu_set_reg(vcpu, kvm_vcpu_sys_get_rt(vcpu), val);
 	__kvm_skip_instr(vcpu);

include/kvm/arm_arch_timer.h

@@ -29,6 +29,11 @@ struct arch_timer_offset {
 	 * structure. If NULL, assume a zero offset.
 	 */
 	u64	*vm_offset;
+	/*
+	 * If set, pointer to one of the offsets in the vcpu's sysreg
+	 * array. If NULL, assume a zero offset.
+	 */
+	u64	*vcpu_offset;
 };
 
 struct arch_timer_vm_data {