diff --git a/driver/linux b/driver/linux
index ece5ef4..01390ae 160000
--- a/driver/linux
+++ b/driver/linux
@@ -1 +1 @@
-Subproject commit ece5ef4a217938841e3157d258212c6d29e3a326
+Subproject commit 01390ae4e344f1363f1c169e3430fa3e72cafaee
diff --git a/inc/hf/arch/cpu.h b/inc/hf/arch/cpu.h
index ccacbbb..384da06 100644
--- a/inc/hf/arch/cpu.h
+++ b/inc/hf/arch/cpu.h
@@ -24,6 +24,8 @@
 
 #include "hf/addr.h"
 
+#include "vmapi/hf/spci.h"
+
 /**
  * Disables interrutps.
  */
@@ -38,7 +40,7 @@
  * Reset the register values other than the PC and argument which are set with
  * `arch_regs_set_pc_arg()`.
  */
-void arch_regs_reset(struct arch_regs *r, bool is_primary, uint64_t vm_id,
+void arch_regs_reset(struct arch_regs *r, bool is_primary, spci_vm_id_t vm_id,
 		     uint64_t vcpu_id, paddr_t table);
 
 /**
diff --git a/inc/hf/vm.h b/inc/hf/vm.h
index 9548f2b..e35edc6 100644
--- a/inc/hf/vm.h
+++ b/inc/hf/vm.h
@@ -73,7 +73,7 @@
 };
 
 struct vm {
-	uint32_t id;
+	spci_vm_id_t id;
 	/** See api.c for the partial ordering on locks. */
 	struct spinlock lock;
 	uint32_t vcpu_count;
diff --git a/inc/vmapi/hf/abi.h b/inc/vmapi/hf/abi.h
index 55c7491..a625a8e 100644
--- a/inc/vmapi/hf/abi.h
+++ b/inc/vmapi/hf/abi.h
@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include "hf/spci.h"
 #include "hf/types.h"
 
 enum hf_vcpu_run_code {
@@ -88,11 +89,11 @@
 	enum hf_vcpu_run_code code;
 	union {
 		struct {
-			uint32_t vm_id;
+			spci_vm_id_t vm_id;
 			uint16_t vcpu;
 		} wake_up;
 		struct {
-			uint16_t vm_id;
+			spci_vm_id_t vm_id;
 		} message;
 		struct {
 			uint64_t ns;
diff --git a/inc/vmapi/hf/call.h b/inc/vmapi/hf/call.h
index 69296d6..239244f 100644
--- a/inc/vmapi/hf/call.h
+++ b/inc/vmapi/hf/call.h
@@ -48,7 +48,7 @@
 /**
  * Returns the VM's own ID.
  */
-static inline uint32_t hf_vm_get_id(void)
+static inline spci_vm_id_t hf_vm_get_id(void)
 {
 	return hf_call(HF_VM_GET_ID, 0, 0, 0);
 }
@@ -64,7 +64,7 @@
 /**
  * Returns the number of VCPUs configured in the given secondary VM.
  */
-static inline int64_t hf_vcpu_get_count(uint32_t vm_id)
+static inline int64_t hf_vcpu_get_count(spci_vm_id_t vm_id)
 {
 	return hf_call(HF_VCPU_GET_COUNT, vm_id, 0, 0);
 }
@@ -74,7 +74,7 @@
  *
  * Returns an hf_vcpu_run_return struct telling the scheduler what to do next.
  */
-static inline struct hf_vcpu_run_return hf_vcpu_run(uint32_t vm_id,
+static inline struct hf_vcpu_run_return hf_vcpu_run(spci_vm_id_t vm_id,
 						    uint32_t vcpu_idx)
 {
 	return hf_vcpu_run_return_decode(
@@ -179,7 +179,7 @@
  * Returns -1 on failure or if there are no waiters; the VM id of the next
  * waiter otherwise.
  */
-static inline int64_t hf_mailbox_waiter_get(uint32_t vm_id)
+static inline int64_t hf_mailbox_waiter_get(spci_vm_id_t vm_id)
 {
 	return hf_call(HF_MAILBOX_WAITER_GET, vm_id, 0, 0);
 }
@@ -217,7 +217,7 @@
  *  - 1 if it was called by the primary VM and the primary VM now needs to wake
  *    up or kick the target vCPU.
  */
-static inline int64_t hf_interrupt_inject(uint32_t target_vm_id,
+static inline int64_t hf_interrupt_inject(spci_vm_id_t target_vm_id,
 					  uint32_t target_vcpu_idx,
 					  uint32_t intid)
 {
@@ -233,7 +233,7 @@
  * TODO: replace this with a better API once we have decided what that should
  *       look like.
  */
-static inline int64_t hf_share_memory(uint32_t vm_id, hf_ipaddr_t addr,
+static inline int64_t hf_share_memory(spci_vm_id_t vm_id, hf_ipaddr_t addr,
 				      size_t size, enum hf_share share)
 {
 	return hf_call(HF_SHARE_MEMORY, (((uint64_t)vm_id) << 32) | share, addr,
diff --git a/src/abi_test.cc b/src/abi_test.cc
index ed060e8..204472d 100644
--- a/src/abi_test.cc
+++ b/src/abi_test.cc
@@ -151,9 +151,9 @@
 {
 	struct hf_vcpu_run_return res = dirty_vcpu_run_return();
 	res.code = HF_VCPU_RUN_WAKE_UP;
-	res.wake_up.vm_id = 0x12345678;
+	res.wake_up.vm_id = 0x1234;
 	res.wake_up.vcpu = 0xabcd;
-	EXPECT_THAT(hf_vcpu_run_return_encode(res), Eq(0x12345678abcd0004));
+	EXPECT_THAT(hf_vcpu_run_return_encode(res), Eq(0x1234abcd0004));
 }
 
 /**
@@ -162,9 +162,9 @@
 TEST(abi, hf_vcpu_run_return_decode_wake_up)
 {
 	struct hf_vcpu_run_return res =
-		hf_vcpu_run_return_decode(0xbeefd00df00daf04);
+		hf_vcpu_run_return_decode(0xbeeff00daf04);
 	EXPECT_THAT(res.code, Eq(HF_VCPU_RUN_WAKE_UP));
-	EXPECT_THAT(res.wake_up.vm_id, Eq(0xbeefd00d));
+	EXPECT_THAT(res.wake_up.vm_id, Eq(0xbeef));
 	EXPECT_THAT(res.wake_up.vcpu, Eq(0xf00d));
 }
 
diff --git a/src/arch/aarch64/cpu.c b/src/arch/aarch64/cpu.c
index bfe9c42..dae0fd8 100644
--- a/src/arch/aarch64/cpu.c
+++ b/src/arch/aarch64/cpu.c
@@ -47,7 +47,7 @@
 #endif
 }
 
-void arch_regs_reset(struct arch_regs *r, bool is_primary, uint64_t vm_id,
+void arch_regs_reset(struct arch_regs *r, bool is_primary, spci_vm_id_t vm_id,
 		     uint64_t vcpu_id, paddr_t table)
 {
 	uintreg_t pc = r->pc;
@@ -90,7 +90,7 @@
 	r->lazy.hcr_el2 = hcr;
 	r->lazy.cptr_el2 = cptr;
 	r->lazy.cnthctl_el2 = cnthctl;
-	r->lazy.vttbr_el2 = pa_addr(table) | (vm_id << 48);
+	r->lazy.vttbr_el2 = pa_addr(table) | ((uint64_t)vm_id << 48);
 	r->lazy.vmpidr_el2 = vcpu_id;
 	/* TODO: Use constant here. */
 	r->spsr = 5 |	 /* M bits, set to EL1h. */
diff --git a/src/arch/fake/hypervisor/cpu.c b/src/arch/fake/hypervisor/cpu.c
index 0f206fc..8542dbc 100644
--- a/src/arch/fake/hypervisor/cpu.c
+++ b/src/arch/fake/hypervisor/cpu.c
@@ -26,7 +26,7 @@
 	/* TODO */
 }
 
-void arch_regs_reset(struct arch_regs *r, bool is_primary, uint64_t vm_id,
+void arch_regs_reset(struct arch_regs *r, bool is_primary, spci_vm_id_t vm_id,
 		     uint64_t vcpu_id, paddr_t table)
 {
 	/* TODO */
diff --git a/test/vmapi/primary_only/primary_only.c b/test/vmapi/primary_only/primary_only.c
index 984a846..3125b20 100644
--- a/test/vmapi/primary_only/primary_only.c
+++ b/test/vmapi/primary_only/primary_only.c
@@ -63,7 +63,7 @@
  */
 TEST(hf_vcpu_get_count, large_invalid_vm_index)
 {
-	EXPECT_EQ(hf_vcpu_get_count(0xffffffff), -1);
+	EXPECT_EQ(hf_vcpu_get_count(0xffff), -1);
 }
 
 /**
diff --git a/test/vmapi/primary_with_secondaries/mailbox.c b/test/vmapi/primary_with_secondaries/mailbox.c
index 28eda77..adb4ba2 100644
--- a/test/vmapi/primary_with_secondaries/mailbox.c
+++ b/test/vmapi/primary_with_secondaries/mailbox.c
@@ -159,14 +159,15 @@
 	 * SERVICE_VM0, then to SERVICE_VM1 and finally back to here.
 	 */
 	{
-		uint32_t *chain = (uint32_t *)mb.send->payload;
+		spci_vm_id_t *chain = (spci_vm_id_t *)mb.send->payload;
 		*chain++ = htole32(SERVICE_VM1);
 		*chain++ = htole32(HF_PRIMARY_VM_ID);
-		memcpy_s(chain, SPCI_MSG_PAYLOAD_MAX - (2 * sizeof(uint32_t)),
+		memcpy_s(chain,
+			 SPCI_MSG_PAYLOAD_MAX - (2 * sizeof(spci_vm_id_t)),
 			 message, sizeof(message));
 
 		spci_message_init(mb.send,
-				  sizeof(message) + (2 * sizeof(uint32_t)),
+				  sizeof(message) + (2 * sizeof(spci_vm_id_t)),
 				  SERVICE_VM0, HF_PRIMARY_VM_ID);
 		EXPECT_EQ(spci_msg_send(0), 0);
 	}
@@ -181,7 +182,7 @@
 	run_res = hf_vcpu_run(SERVICE_VM1, 0);
 	EXPECT_EQ(run_res.code, HF_VCPU_RUN_MESSAGE);
 
-	/* Ensure the message is in tact. */
+	/* Ensure the message is intact. */
 	EXPECT_EQ(run_res.message.vm_id, HF_PRIMARY_VM_ID);
 	EXPECT_EQ(mb.recv->length, sizeof(message));
 	EXPECT_EQ(memcmp(mb.recv->payload, message, sizeof(message)), 0);
diff --git a/test/vmapi/primary_with_secondaries/no_services.c b/test/vmapi/primary_with_secondaries/no_services.c
index b6e27be..98687ec 100644
--- a/test/vmapi/primary_with_secondaries/no_services.c
+++ b/test/vmapi/primary_with_secondaries/no_services.c
@@ -64,7 +64,7 @@
  */
 TEST(hf_vcpu_get_count, large_invalid_vm_index)
 {
-	EXPECT_EQ(hf_vcpu_get_count(0xffffffff), -1);
+	EXPECT_EQ(hf_vcpu_get_count(0xffff), -1);
 }
 
 /**
diff --git a/test/vmapi/primary_with_secondaries/services/interruptible.c b/test/vmapi/primary_with_secondaries/services/interruptible.c
index 3df67eb..5a04515 100644
--- a/test/vmapi/primary_with_secondaries/services/interruptible.c
+++ b/test/vmapi/primary_with_secondaries/services/interruptible.c
@@ -64,7 +64,7 @@
 
 TEST_SERVICE(interruptible)
 {
-	uint32_t this_vm_id = hf_vm_get_id();
+	spci_vm_id_t this_vm_id = hf_vm_get_id();
 	struct spci_message *recv_buf = SERVICE_RECV_BUFFER();
 
 	exception_setup(irq);
diff --git a/test/vmapi/primary_with_secondaries/services/relay.c b/test/vmapi/primary_with_secondaries/services/relay.c
index 413fbbf..5ccda5f 100644
--- a/test/vmapi/primary_with_secondaries/services/relay.c
+++ b/test/vmapi/primary_with_secondaries/services/relay.c
@@ -30,8 +30,8 @@
 	 * message so multiple IDs can be places at the start of the message.
 	 */
 	for (;;) {
-		uint32_t *chain;
-		uint32_t next_vm_id;
+		spci_vm_id_t *chain;
+		spci_vm_id_t next_vm_id;
 		void *next_message;
 		uint32_t next_message_size;
 
@@ -41,12 +41,12 @@
 		/* Prepare to relay the message. */
 		struct spci_message *recv_buf = SERVICE_RECV_BUFFER();
 		struct spci_message *send_buf = SERVICE_SEND_BUFFER();
-		ASSERT_GE(recv_buf->length, sizeof(uint32_t));
+		ASSERT_GE(recv_buf->length, sizeof(spci_vm_id_t));
 
-		chain = (uint32_t *)recv_buf->payload;
-		next_vm_id = le32toh(*chain);
+		chain = (spci_vm_id_t *)recv_buf->payload;
+		next_vm_id = le16toh(*chain);
 		next_message = chain + 1;
-		next_message_size = recv_buf->length - sizeof(uint32_t);
+		next_message_size = recv_buf->length - sizeof(spci_vm_id_t);
 
 		/* Send the message to the next stage. */
 		memcpy_s(send_buf->payload, SPCI_MSG_PAYLOAD_MAX, next_message,