diff --git a/.clang-format b/.clang-format
index 10dc5a9a61b3..3f11c8901b43 100644
--- a/.clang-format
+++ b/.clang-format
@@ -109,6 +109,7 @@ ForEachMacros:
   - 'css_for_each_child'
   - 'css_for_each_descendant_post'
   - 'css_for_each_descendant_pre'
+  - 'cxl_for_each_cmd'
   - 'device_for_each_child_node'
   - 'dma_fence_chain_for_each'
   - 'do_for_each_ftrace_op'
diff --git a/Documentation/cxl/memory-devices.rst b/Documentation/cxl/memory-devices.rst
index 5f723c25382b..ec54674b3822 100644
--- a/Documentation/cxl/memory-devices.rst
+++ b/Documentation/cxl/memory-devices.rst
@@ -32,6 +32,15 @@ CXL Memory Device
 .. kernel-doc:: drivers/cxl/mem.c
    :internal:
 
+CXL IOCTL Interface
+-------------------
+
+.. kernel-doc:: include/uapi/linux/cxl_mem.h
+   :doc: UAPI
+
+.. kernel-doc:: include/uapi/linux/cxl_mem.h
+   :internal:
+
 External Interfaces
 ===================
 
diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
index a4c75a28c839..6eb8e634664d 100644
--- a/Documentation/userspace-api/ioctl/ioctl-number.rst
+++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
@@ -352,6 +352,7 @@ Code  Seq#    Include File                                           Comments
                                                                      <mailto:michael.klein@puffin.lb.shuttle.de>
 0xCC  00-0F  drivers/misc/ibmvmc.h                                   pseries VMC driver
 0xCD  01     linux/reiserfs_fs.h
+0xCE  01-02  uapi/linux/cxl_mem.h                                    Compute Express Link Memory Devices
 0xCF  02     fs/cifs/ioctl.c
 0xDB  00-0F  drivers/char/mwave/mwavepub.h
 0xDD  00-3F                                                          ZFCP device driver see drivers/s390/scsi/
diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
index da2bf941fe92..d4eb3f5b9469 100644
--- a/drivers/cxl/mem.c
+++ b/drivers/cxl/mem.c
@@ -7,6 +7,7 @@
 #include <linux/idr.h>
 #include <linux/pci.h>
 #include <linux/io.h>
+#include <uapi/linux/cxl_mem.h>
 #include "acpi.h"
 #include "pci.h"
 #include "cxl.h"
@@ -41,6 +42,7 @@
 #define CXL_MAILBOX_TIMEOUT_US 2000
 
 enum opcode {
+	CXL_MBOX_OP_INVALID	= 0x0000,
 	CXL_MBOX_OP_IDENTIFY    = 0x4000,
 	CXL_MBOX_OP_MAX         = 0x10000
 };
@@ -82,6 +84,90 @@ static DEFINE_IDR(cxl_mem_idr);
 /* protect cxl_mem_idr allocations */
 static DEFINE_MUTEX(cxl_memdev_lock);
 
+#define CXL_CMD(_id, _flags, sin, sout, f)                                     \
+	[CXL_MEM_COMMAND_ID_##_id] = {                                         \
+		{                                                              \
+			.id = CXL_MEM_COMMAND_ID_##_id,                        \
+			.flags = CXL_MEM_COMMAND_FLAG_##_flags,                \
+			.size_in = sin,                                        \
+			.size_out = sout,                                      \
+		},                                                             \
+			.flags = CXL_CMD_INTERNAL_FLAG_##f,                    \
+			.opcode = CXL_MBOX_OP_##_id,                           \
+	}
+
+/**
+ * struct cxl_mem_command - Driver representation of a memory device command
+ * @info: Command information as it exists for the UAPI
+ * @opcode: The actual bits used for the mailbox protocol
+ * @flags: Set of flags reflecting the state of the command.
+ *
+ *  * %CXL_CMD_INTERNAL_FLAG_HIDDEN: Command is hidden from userspace. This
+ *    would typically be used for deprecated commands.
+ *  * %CXL_CMD_FLAG_MANDATORY: Hardware must support this command. This flag is
+ *    only used internally by the driver for sanity checking.
+ *
+ * The cxl_mem_command is the driver's internal representation of commands that
+ * are supported by the driver. Some of these commands may not be supported by
+ * the hardware. The driver will use @info to validate the fields passed in by
+ * the user then submit the @opcode to the hardware.
+ *
+ * See struct cxl_command_info.
+ */
+struct cxl_mem_command {
+	const struct cxl_command_info info;
+	enum opcode opcode;
+	u32 flags;
+#define CXL_CMD_INTERNAL_FLAG_NONE 0
+#define CXL_CMD_INTERNAL_FLAG_HIDDEN BIT(0)
+#define CXL_CMD_INTERNAL_FLAG_MANDATORY BIT(1)
+};
+
+/*
+ * This table defines the supported mailbox commands for the driver. This table
+ * is made up of a UAPI structure. Non-negative values as parameters in the
+ * table will be validated against the user's input. For example, if size_in is
+ * 0, and the user passed in 1, it is an error.
+ */
+static struct cxl_mem_command mem_commands[] = {
+	CXL_CMD(INVALID, KERNEL, 0, 0, HIDDEN),
+	CXL_CMD(IDENTIFY, NONE, 0, 0x43, MANDATORY),
+};
+
+#define cxl_for_each_cmd(cmd)                                                  \
+	for ((cmd) = &mem_commands[0];                                         \
+	     ((cmd) - mem_commands) < ARRAY_SIZE(mem_commands); (cmd)++)
+
+static inline int cxl_cmd_index(const struct cxl_mem_command *c)
+{
+	if (unlikely(c >= mem_commands &&
+		     c < (mem_commands + sizeof(mem_commands)))) {
+		struct cxl_mem_command *p;
+
+		cxl_for_each_cmd(p)
+			if (p->opcode == c->opcode &&
+			    p->flags == c->flags)
+				return p - mem_commands;
+	} else {
+		return c - mem_commands;
+	}
+
+	WARN_ON_ONCE(1);
+	return 0;
+}
+
+static inline struct cxl_mem_command *cxl_mem_find_command(u16 opcode)
+{
+	struct cxl_mem_command *c;
+
+	cxl_for_each_cmd(c) {
+		if (c->opcode == opcode)
+			return c;
+	}
+
+	return NULL;
+}
+
 static int cxl_mem_wait_for_doorbell(struct cxl_mem *cxlm)
 {
 	const int timeout = msecs_to_jiffies(CXL_MAILBOX_TIMEOUT_US);
@@ -140,6 +226,7 @@ static void cxl_mem_mbox_timeout(struct cxl_mem *cxlm,
 static int cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
 				 struct mbox_cmd *mbox_cmd)
 {
+	const struct cxl_mem_command *cmd;
 	u64 cmd_reg, status_reg;
 	size_t out_len;
 	int rc;
@@ -158,6 +245,13 @@ static int cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
 	 *   8. If output payload is non-empty, host reads Command Payload Registers
 	 */
 
+	cmd = cxl_mem_find_command(mbox_cmd->opcode);
+	if (!cmd) {
+		dev_info(&cxlm->pdev->dev,
+			 "Unknown opcode 0x%04x being sent to hardware\n",
+			 mbox_cmd->opcode);
+	}
+
 	/* #1 */
 	WARN_ON(cxl_doorbell_busy(cxlm));
 
@@ -200,6 +294,20 @@ static int cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
 	out_len = CXL_GET_FIELD(cmd_reg, CXLDEV_MB_CMD_PAYLOAD_LENGTH);
 	mbox_cmd->size_out = out_len;
 
+	/*
+	 * If the command had a fixed size output, but the hardware did
+	 * something unexpected, just print an error and move on. It would be
+	 * worth sending a bug report.
+	 */
+	if (cmd && cmd->info.size_out >= 0 &&
+	    mbox_cmd->size_out != cmd->info.size_out) {
+		bool too_big = mbox_cmd->size_out > cmd->info.size_out;
+
+		dev_err(&cxlm->pdev->dev,
+			"payload was %s than driver expectations\n",
+			too_big ? "larger" : "smaller");
+	}
+
 	/* #8 */
 	if (out_len && mbox_cmd->payload)
 		memcpy_fromio(mbox_cmd->payload, cxl_payload_regs(cxlm),
@@ -323,8 +431,56 @@ static int cxl_mem_release(struct inode *inode, struct file *file)
 	return 0;
 }
 
+static int cxl_mem_count_commands(void)
+{
+	struct cxl_mem_command *c;
+	int n = 0;
+
+	cxl_for_each_cmd(c) {
+		if (c->flags & CXL_CMD_INTERNAL_FLAG_HIDDEN)
+			continue;
+		n++;
+	}
+
+	return n;
+}
+
 static long cxl_mem_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
+	struct cxl_memdev *cxlmd = file->private_data;
+	struct device *dev = &cxlmd->dev;
+
+	if (cmd == CXL_MEM_QUERY_COMMANDS) {
+		struct cxl_mem_query_commands __user *q = (void __user *)arg;
+		struct cxl_mem_command *cmd;
+		u32 n_commands;
+		int j = 0;
+
+		dev_dbg(dev, "Query IOCTL\n");
+
+		if (get_user(n_commands, (u32 __user *)arg))
+			return -EFAULT;
+
+		/* returns the total number if 0 elements are requested. */
+		if (n_commands == 0)
+			return put_user(cxl_mem_count_commands(),
+					(u32 __user *)arg);
+
+		/* otherwise, return max(n_commands, total commands) */
+		cxl_for_each_cmd(cmd) {
+			const struct cxl_command_info *info = &cmd->info;
+
+			if (cmd->flags & CXL_CMD_INTERNAL_FLAG_HIDDEN)
+				continue;
+
+			if (copy_to_user(&q->commands[j++], info, sizeof(*info)))
+				return -EFAULT;
+
+			if (j == n_commands)
+				break;
+		}
+	}
+
 	return -ENOTTY;
 }
 
diff --git a/include/uapi/linux/cxl_mem.h b/include/uapi/linux/cxl_mem.h
new file mode 100644
index 000000000000..847f825bbe18
--- /dev/null
+++ b/include/uapi/linux/cxl_mem.h
@@ -0,0 +1,117 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/*
+ * CXL IOCTLs for Memory Devices
+ */
+
+#ifndef _UAPI_CXL_MEM_H_
+#define _UAPI_CXL_MEM_H_
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+/**
+ * DOC: UAPI
+ *
+ * CXL memory devices expose UAPI to have a standard user interface.
+ * Userspace can refer to these structure definitions and UAPI formats
+ * to communicate to driver. The commands themselves are somewhat obfuscated
+ * with macro magic. They have the form CXL_MEM_COMMAND_ID_<name>.
+ *
+ * For example "CXL_MEM_COMMAND_ID_INVALID"
+ *
+ * Not all of all commands that the driver supports are always available for use
+ * by userspace. Userspace must check the results from the QUERY command in
+ * order to determine the live set of commands.
+ */
+
+#define CXL_MEM_QUERY_COMMANDS _IOR(0xCE, 1, struct cxl_mem_query_commands)
+
+#undef CMDS
+#define CMDS                                                                   \
+	C(INVALID,	"Invalid Command"),                                    \
+	C(IDENTIFY,	"Identify Command"),                                   \
+	C(MAX,		"Last command")
+#undef C
+#define C(a, b) CXL_MEM_COMMAND_ID_##a
+
+enum { CMDS };
+
+/**
+ * struct cxl_command_info - Command information returned from a query.
+ * @id: ID number for the command.
+ * @flags: Flags that specify command behavior.
+ *
+ *  * %CXL_MEM_COMMAND_FLAG_KERNEL: This command is reserved for exclusive
+ *    kernel use.
+ *  * %CXL_MEM_COMMAND_FLAG_MUTEX: This command may require coordination with
+ *    the kernel in order to complete successfully.
+ *
+ * @size_in: Expected input size, or -1 if variable length.
+ * @size_out: Expected output size, or -1 if variable length.
+ *
+ * Represents a single command that is supported by both the driver and the
+ * hardware. The is returned as part of an array from the query ioctl. The
+ * following would be a command named "foobar" that takes a variable length
+ * input and returns 0 bytes of output.
+ *
+ *  - @id = 10
+ *  - @flags = CXL_MEM_COMMAND_FLAG_MUTEX
+ *  - @size_in = -1
+ *  - @size_out = 0
+ *
+ * See struct cxl_mem_query_commands.
+ */
+struct cxl_command_info {
+	__u32 id;
+
+	__u32 flags;
+#define CXL_MEM_COMMAND_FLAG_NONE 0
+#define CXL_MEM_COMMAND_FLAG_KERNEL BIT(0)
+#define CXL_MEM_COMMAND_FLAG_MUTEX BIT(1)
+
+	__s32 size_in;
+	__s32 size_out;
+};
+
+/**
+ * struct cxl_mem_query_commands - Query supported commands.
+ * @n_commands: In/out parameter. When @n_commands is > 0, the driver will
+ *		return min(num_support_commands, n_commands). When @n_commands
+ *		is 0, driver will return the number of total supported commands.
+ * @rsvd: Reserved for future use.
+ * @commands: Output array of supported commands. This array must be allocated
+ *            by userspace to be at least min(num_support_commands, @n_commands)
+ *
+ * Allow userspace to query the available commands supported by both the driver,
+ * and the hardware. Commands that aren't supported by either the driver, or the
+ * hardware are not returned in the query.
+ *
+ * Examples:
+ *
+ *  - { .n_commands = 0 } // Get number of supported commands
+ *  - { .n_commands = 15, .commands = buf } // Return first 15 (or less)
+ *    supported commands
+ *
+ *  See struct cxl_command_info.
+ */
+struct cxl_mem_query_commands {
+	/*
+	 * Input: Number of commands to return (space allocated by user)
+	 * Output: Number of commands supported by the driver/hardware
+	 *
+	 * If n_commands is 0, kernel will only return number of commands and
+	 * not try to populate commands[], thus allowing userspace to know how
+	 * much space to allocate
+	 */
+	__u32 n_commands;
+	__u32 rsvd;
+
+	struct cxl_command_info __user commands[]; /* out: supported commands */
+};
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif
