diff --git a/Documentation/virt/kvm/kvmi.rst b/Documentation/virt/kvm/kvmi.rst
index af685e837b4b..4aa77ae0c3c5 100644
--- a/Documentation/virt/kvm/kvmi.rst
+++ b/Documentation/virt/kvm/kvmi.rst
@@ -64,6 +64,85 @@ an answer from its peer indefinitely or until the guest is powered off
 can really continue normal execution depends on whether the introspection
 tool has made any modifications that require an active KVMI channel.
 
+All messages (commands or events) have a common header::
+
+	struct kvmi_msg_hdr {
+		__u16 id;
+		__u16 size;
+		__u32 seq;
+	};
+
+The replies have the same header, with the sequence number (``seq``)
+and message id (``id``) matching the command/event.
+
+After ``kvmi_msg_hdr``, ``id`` specific data of ``size`` bytes will
+follow.
+
+The message header and its data must be sent with one ``sendmsg()`` call
+to the socket. This simplifies the receiver loop and avoids
+the reconstruction of messages on the other side.
+
+The wire protocol uses the host native byte-order. The introspection tool
+must check this during the handshake and do the necessary conversion.
+
+A command reply begins with::
+
+	struct kvmi_error_code {
+		__s32 err;
+		__u32 padding;
+	}
+
+followed by the command specific data if the error code ``err`` is zero.
+
+The error code -KVM_ENOSYS is returned for unsupported commands.
+
+The error code -KVM_EPERM is returned for disallowed commands (see **Hooking**).
+
+The error code is related to the message processing, including unsupported
+commands. For all the other errors (incomplete messages, wrong sequence
+numbers, socket errors etc.) the socket will be closed. The device
+manager should reconnect.
+
+While all commands will have a reply as soon as possible, the replies
+to events will probably be delayed until a set of (new) commands will
+complete::
+
+   Host kernel               Tool
+   -----------               ----
+   event 1 ->
+                             <- command 1
+   command 1 reply ->
+                             <- command 2
+   command 2 reply ->
+                             <- event 1 reply
+
+If both ends send a message at the same time::
+
+   Host kernel               Tool
+   -----------               ----
+   event X ->                <- command X
+
+the host kernel will reply to 'command X', regardless of the receive time
+(before or after the 'event X' was sent).
+
+As it can be seen below, the wire protocol specifies occasional padding. This
+is to permit working with the data by directly using C structures or to round
+the structure size to a multiple of 8 bytes (64bit) to improve the copy
+operations that happen during ``recvmsg()`` or ``sendmsg()``. The members
+should have the native alignment of the host (4 bytes on x86). All padding
+must be initialized with zero otherwise the respective commands will fail
+with -KVM_EINVAL.
+
+To describe the commands/events, we reuse some conventions from api.txt:
+
+  - Architectures: which instruction set architectures provide this command/event
+
+  - Versions: which versions provide this command/event
+
+  - Parameters: incoming message data
+
+  - Returns: outgoing/reply message data
+
 Handshake
 ---------
 
@@ -99,6 +178,13 @@ commands/events) to KVM, and forget about it. It will be notified by
 KVM when the introspection tool closes the file handle (in case of
 errors), and should reinitiate the handshake.
 
+Once the file handle reaches KVM, the introspection tool should
+use the *KVMI_GET_VERSION* command to get the API version and/or the
+*KVMI_VM_CHECK_COMMAND* and *KVMI_VM_CHECK_EVENT* commands to see which
+commands/events are allowed for this guest. The error code -KVM_EPERM
+will be returned if the introspection tool uses a command or enables an
+event which is disallowed.
+
 Unhooking
 ---------
 
diff --git a/include/uapi/linux/kvmi.h b/include/uapi/linux/kvmi.h
index d7b18ffef4fa..7620fdbf4749 100644
--- a/include/uapi/linux/kvmi.h
+++ b/include/uapi/linux/kvmi.h
@@ -18,4 +18,19 @@ enum {
 	KVMI_NUM_EVENTS
 };
 
+struct kvmi_msg_hdr {
+	__u16 id;
+	__u16 size;
+	__u32 seq;
+};
+
+enum {
+	KVMI_MSG_SIZE = (4096 * 2 - sizeof(struct kvmi_msg_hdr))
+};
+
+struct kvmi_error_code {
+	__s32 err;
+	__u32 padding;
+};
+
 #endif /* _UAPI__LINUX_KVMI_H */
diff --git a/tools/testing/selftests/kvm/x86_64/kvmi_test.c b/tools/testing/selftests/kvm/x86_64/kvmi_test.c
index ea411611e296..1793582b7e10 100644
--- a/tools/testing/selftests/kvm/x86_64/kvmi_test.c
+++ b/tools/testing/selftests/kvm/x86_64/kvmi_test.c
@@ -15,6 +15,9 @@
 #include "processor.h"
 #include "../lib/kvm_util_internal.h"
 
+#include "linux/kvm_para.h"
+#include "linux/kvmi.h"
+
 #define VCPU_ID         5
 
 static int socket_pair[2];
@@ -80,10 +83,107 @@ static void unhook_introspection(struct kvm_vm *vm)
 		errno, strerror(errno));
 }
 
+static void receive_data(void *dest, size_t size)
+{
+	ssize_t r;
+
+	r = recv(Userspace_socket, dest, size, MSG_WAITALL);
+	TEST_ASSERT(r == size,
+		"recv() failed, expected %d, result %d, errno %d (%s)\n",
+		size, r, errno, strerror(errno));
+}
+
+static int receive_cmd_reply(struct kvmi_msg_hdr *req, void *rpl,
+			     size_t rpl_size)
+{
+	struct kvmi_msg_hdr hdr;
+	struct kvmi_error_code ec;
+
+	receive_data(&hdr, sizeof(hdr));
+
+	TEST_ASSERT(hdr.seq == req->seq,
+		"Unexpected messages sequence 0x%x, expected 0x%x\n",
+		hdr.seq, req->seq);
+
+	TEST_ASSERT(hdr.size >= sizeof(ec),
+		"Invalid message size %d, expected %d bytes (at least)\n",
+		hdr.size, sizeof(ec));
+
+	receive_data(&ec, sizeof(ec));
+
+	if (ec.err) {
+		TEST_ASSERT(hdr.size == sizeof(ec),
+			"Invalid command reply on error\n");
+	} else {
+		TEST_ASSERT(hdr.size == sizeof(ec) + rpl_size,
+			"Invalid command reply\n");
+
+		if (rpl && rpl_size)
+			receive_data(rpl, rpl_size);
+	}
+
+	return ec.err;
+}
+
+static unsigned int new_seq(void)
+{
+	static unsigned int seq;
+
+	return seq++;
+}
+
+static void send_message(int msg_id, struct kvmi_msg_hdr *hdr, size_t size)
+{
+	ssize_t r;
+
+	hdr->id = msg_id;
+	hdr->seq = new_seq();
+	hdr->size = size - sizeof(*hdr);
+
+	r = send(Userspace_socket, hdr, size, 0);
+	TEST_ASSERT(r == size,
+		"send() failed, sending %d, result %d, errno %d (%s)\n",
+		size, r, errno, strerror(errno));
+}
+
+static const char *kvm_strerror(int error)
+{
+	switch (error) {
+	case KVM_ENOSYS:
+		return "Invalid system call number";
+	case KVM_EOPNOTSUPP:
+		return "Operation not supported on transport endpoint";
+	default:
+		return strerror(error);
+	}
+}
+
+static int do_command(int cmd_id, struct kvmi_msg_hdr *req,
+		      size_t req_size, void *rpl, size_t rpl_size)
+{
+	send_message(cmd_id, req, req_size);
+	return receive_cmd_reply(req, rpl, rpl_size);
+}
+
+static void test_cmd_invalid(void)
+{
+	int invalid_msg_id = 0xffff;
+	struct kvmi_msg_hdr req;
+	int r;
+
+	r = do_command(invalid_msg_id, &req, sizeof(req), NULL, 0);
+	TEST_ASSERT(r == -KVM_ENOSYS,
+		"Invalid command didn't failed with KVM_ENOSYS, error %d (%s)\n",
+		-r, kvm_strerror(-r));
+}
+
 static void test_introspection(struct kvm_vm *vm)
 {
 	setup_socket();
 	hook_introspection(vm);
+
+	test_cmd_invalid();
+
 	unhook_introspection(vm);
 }
 
diff --git a/virt/kvm/introspection/kvmi.c b/virt/kvm/introspection/kvmi.c
index 791d2536415e..706372e9e56a 100644
--- a/virt/kvm/introspection/kvmi.c
+++ b/virt/kvm/introspection/kvmi.c
@@ -8,13 +8,54 @@
 #include "kvmi_int.h"
 #include <linux/kthread.h>
 
-int kvmi_init(void)
+static struct kmem_cache *msg_cache;
+
+void *kvmi_msg_alloc(void)
+{
+	return kmem_cache_zalloc(msg_cache, GFP_KERNEL);
+}
+
+void *kvmi_msg_alloc_check(size_t size)
+{
+	if (size > KVMI_MSG_SIZE_ALLOC)
+		return NULL;
+	return kvmi_msg_alloc();
+}
+
+void kvmi_msg_free(void *addr)
+{
+	if (addr)
+		kmem_cache_free(msg_cache, addr);
+}
+
+static void kvmi_cache_destroy(void)
 {
+	kmem_cache_destroy(msg_cache);
+	msg_cache = NULL;
+}
+
+static int kvmi_cache_create(void)
+{
+	msg_cache = kmem_cache_create("kvmi_msg", KVMI_MSG_SIZE_ALLOC,
+				      4096, SLAB_ACCOUNT, NULL);
+
+	if (!msg_cache) {
+		kvmi_cache_destroy();
+
+		return -1;
+	}
+
 	return 0;
 }
 
+int kvmi_init(void)
+{
+	return kvmi_cache_create();
+}
+
 void kvmi_uninit(void)
 {
+	kvmi_cache_destroy();
 }
 
 static void free_kvmi(struct kvm *kvm)
diff --git a/virt/kvm/introspection/kvmi_int.h b/virt/kvm/introspection/kvmi_int.h
index c5cf40d03d68..46ba90cb5e66 100644
--- a/virt/kvm/introspection/kvmi_int.h
+++ b/virt/kvm/introspection/kvmi_int.h
@@ -16,6 +16,8 @@
 #define kvmi_err(kvmi, fmt, ...) \
 	kvm_info("%pU ERROR: " fmt, &kvmi->uuid, ## __VA_ARGS__)
 
+#define KVMI_MSG_SIZE_ALLOC (sizeof(struct kvmi_msg_hdr) + KVMI_MSG_SIZE)
+
 #define KVMI_KNOWN_EVENTS 0
 
 #define KVMI_KNOWN_COMMANDS 0
@@ -28,4 +30,9 @@ void kvmi_sock_shutdown(struct kvm_introspection *kvmi);
 void kvmi_sock_put(struct kvm_introspection *kvmi);
 bool kvmi_msg_process(struct kvm_introspection *kvmi);
 
+/* kvmi.c */
+void *kvmi_msg_alloc(void);
+void *kvmi_msg_alloc_check(size_t size);
+void kvmi_msg_free(void *addr);
+
 #endif
diff --git a/virt/kvm/introspection/kvmi_msg.c b/virt/kvm/introspection/kvmi_msg.c
index f9e66274fb43..ed095fcf50bc 100644
--- a/virt/kvm/introspection/kvmi_msg.c
+++ b/virt/kvm/introspection/kvmi_msg.c
@@ -8,6 +8,19 @@
 #include <linux/net.h>
 #include "kvmi_int.h"
 
+static const char *const msg_IDs[] = {
+};
+
+static bool is_known_message(u16 id)
+{
+	return id < ARRAY_SIZE(msg_IDs) && msg_IDs[id];
+}
+
+static const char *id2str(u16 id)
+{
+	return is_known_message(id) ? msg_IDs[id] : "unknown";
+}
+
 bool kvmi_sock_get(struct kvm_introspection *kvmi, int fd)
 {
 	struct socket *sock;
@@ -33,7 +46,216 @@ void kvmi_sock_shutdown(struct kvm_introspection *kvmi)
 	kernel_sock_shutdown(kvmi->sock, SHUT_RDWR);
 }
 
+static int kvmi_sock_read(struct kvm_introspection *kvmi, void *buf,
+			  size_t size)
+{
+	struct kvec i = {
+		.iov_base = buf,
+		.iov_len = size,
+	};
+	struct msghdr m = { };
+	int rc;
+
+	rc = kernel_recvmsg(kvmi->sock, &m, &i, 1, size, MSG_WAITALL);
+
+	if (unlikely(rc != size && rc >= 0))
+		rc = -EPIPE;
+
+	return rc >= 0 ? 0 : rc;
+}
+
+static int kvmi_sock_write(struct kvm_introspection *kvmi, struct kvec *i,
+			   size_t n, size_t size)
+{
+	struct msghdr m = { };
+	int rc;
+
+	rc = kernel_sendmsg(kvmi->sock, &m, i, n, size);
+
+	if (unlikely(rc != size && rc >= 0))
+		rc = -EPIPE;
+
+	return rc >= 0 ? 0 : rc;
+}
+
+static int kvmi_msg_reply(struct kvm_introspection *kvmi,
+			  const struct kvmi_msg_hdr *msg, int err,
+			  const void *rpl, size_t rpl_size)
+{
+	struct kvmi_error_code ec;
+	struct kvmi_msg_hdr h;
+	struct kvec vec[3] = {
+		{ .iov_base = &h, .iov_len = sizeof(h) },
+		{ .iov_base = &ec, .iov_len = sizeof(ec) },
+		{ .iov_base = (void *)rpl, .iov_len = rpl_size },
+	};
+	size_t size = sizeof(h) + sizeof(ec) + (err ? 0 : rpl_size);
+	size_t n = err ? ARRAY_SIZE(vec) - 1 : ARRAY_SIZE(vec);
+
+	memset(&h, 0, sizeof(h));
+	h.id = msg->id;
+	h.seq = msg->seq;
+	h.size = size - sizeof(h);
+
+	memset(&ec, 0, sizeof(ec));
+	ec.err = err;
+
+	return kvmi_sock_write(kvmi, vec, n, size);
+}
+
+static int kvmi_msg_vm_reply(struct kvm_introspection *kvmi,
+			     const struct kvmi_msg_hdr *msg,
+			     int err, const void *rpl,
+			     size_t rpl_size)
+{
+	return kvmi_msg_reply(kvmi, msg, err, rpl, rpl_size);
+}
+
+static bool is_command_allowed(struct kvm_introspection *kvmi, int id)
+{
+	return test_bit(id, kvmi->cmd_allow_mask);
+}
+
+/*
+ * These commands are executed by the receiving thread/worker.
+ */
+static int(*const msg_vm[])(struct kvm_introspection *,
+			    const struct kvmi_msg_hdr *, const void *) = {
+};
+
+static bool is_vm_message(u16 id)
+{
+	return id < ARRAY_SIZE(msg_vm) && !!msg_vm[id];
+}
+
+static bool is_unsupported_message(u16 id)
+{
+	bool supported;
+
+	supported = is_known_message(id) && is_vm_message(id);
+
+	return !supported;
+}
+
+static int kvmi_consume_bytes(struct kvm_introspection *kvmi, size_t bytes)
+{
+	size_t to_read;
+	u8 buf[1024];
+	int err = 0;
+
+	while (bytes && !err) {
+		to_read = min(bytes, sizeof(buf));
+
+		err = kvmi_sock_read(kvmi, buf, to_read);
+
+		bytes -= to_read;
+	}
+
+	return err;
+}
+
+static struct kvmi_msg_hdr *
+kvmi_msg_recv(struct kvm_introspection *kvmi, bool *unsupported)
+{
+	struct kvmi_msg_hdr *msg;
+	int err;
+
+	*unsupported = false;
+
+	msg = kvmi_msg_alloc();
+	if (!msg)
+		goto out_err;
+
+	err = kvmi_sock_read(kvmi, msg, sizeof(*msg));
+	if (err)
+		goto out_err;
+
+	if (msg->size > KVMI_MSG_SIZE)
+		goto out_err_msg;
+
+	if (is_unsupported_message(msg->id)) {
+		if (msg->size && kvmi_consume_bytes(kvmi, msg->size) < 0)
+			goto out_err_msg;
+
+		*unsupported = true;
+		return msg;
+	}
+
+	if (msg->size && kvmi_sock_read(kvmi, msg + 1, msg->size) < 0)
+		goto out_err_msg;
+
+	return msg;
+
+out_err_msg:
+	kvmi_err(kvmi, "%s id %u (%s) size %u\n",
+		 __func__, msg->id, id2str(msg->id), msg->size);
+
+out_err:
+	kvmi_msg_free(msg);
+
+	return NULL;
+}
+
+static int kvmi_msg_dispatch_vm_cmd(struct kvm_introspection *kvmi,
+				    const struct kvmi_msg_hdr *msg)
+{
+	return msg_vm[msg->id](kvmi, msg, msg + 1);
+}
+
+static int kvmi_msg_dispatch(struct kvm_introspection *kvmi,
+			     struct kvmi_msg_hdr *msg, bool *queued)
+{
+	int err;
+
+	err = kvmi_msg_dispatch_vm_cmd(kvmi, msg);
+
+	if (err)
+		kvmi_err(kvmi, "%s: msg id: %u (%s), err: %d\n", __func__,
+			 msg->id, id2str(msg->id), err);
+
+	return err;
+}
+
+static bool is_message_allowed(struct kvm_introspection *kvmi, __u16 id)
+{
+	if (id >= KVMI_NUM_COMMANDS)
+		return false;
+
+	return is_command_allowed(kvmi, id);
+}
+
+static int kvmi_msg_vm_reply_ec(struct kvm_introspection *kvmi,
+				const struct kvmi_msg_hdr *msg, int ec)
+{
+	return kvmi_msg_vm_reply(kvmi, msg, ec, NULL, 0);
+}
+
 bool kvmi_msg_process(struct kvm_introspection *kvmi)
 {
-	return false;
+	struct kvmi_msg_hdr *msg;
+	bool queued = false;
+	bool unsupported;
+	int err = -1;
+
+	msg = kvmi_msg_recv(kvmi, &unsupported);
+	if (!msg)
+		goto out;
+
+	if (unsupported) {
+		err = kvmi_msg_vm_reply_ec(kvmi, msg, -KVM_ENOSYS);
+		goto out;
+	}
+
+	if (!is_message_allowed(kvmi, msg->id)) {
+		err = kvmi_msg_vm_reply_ec(kvmi, msg, -KVM_EPERM);
+		goto out;
+	}
+
+	err = kvmi_msg_dispatch(kvmi, msg, &queued);
+
+out:
+	if (!queued)
+		kvmi_msg_free(msg);
+
+	return err == 0;
 }
