Message-ID: <20210216175314.ut2dn5ujayj57zp2@intel.com>
Date:   Tue, 16 Feb 2021 09:53:14 -0800
From:   Ben Widawsky <ben.widawsky@...el.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     linux-cxl@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-pci@...r.kernel.org, Bjorn Helgaas <helgaas@...nel.org>,
        Chris Browy <cbrowy@...ry-design.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Jon Masters <jcm@...masters.org>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "John Groves (jgroves)" <jgroves@...ron.com>,
        "Kelley, Sean V" <sean.v.kelley@...el.com>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v4 4/9] cxl/mem: Add basic IOCTL interface

On 21-02-16 15:22:23, Jonathan Cameron wrote:
> On Mon, 15 Feb 2021 17:45:33 -0800
> Ben Widawsky <ben.widawsky@...el.com> wrote:
> 
> > Add a straightforward IOCTL that provides a mechanism for userspace to
> > query the supported memory device commands. CXL commands as they appear
> > to userspace are described as part of the UAPI kerneldoc. The command
> > list returned via this IOCTL will contain the full set of commands that
> > the driver supports, however, some of those commands may not be
> > available for use by userspace.
> > 
> > Memory device commands first appear in the CXL 2.0 specification. They
> > are submitted through a mailbox mechanism specified in the CXL 2.0
> > specification.
> > 
> > The send command allows userspace to issue mailbox commands directly to
> > the hardware. The list of available commands to send are the output of
> > the query command. The driver verifies basic properties of the command
> > and possibly inspect the input (or output) payload to determine whether
> > or not the command is allowed (or might taint the kernel).
> > 
> > Reported-by: kernel test robot <lkp@...el.com> # bug in earlier revision
> > Signed-off-by: Ben Widawsky <ben.widawsky@...el.com>
> > Reviewed-by: Dan Williams <dan.j.williams@...el.com> (v2)
> 
> I may be missreading this but I think the logic to ensure commands
> using a variable sized buffer have enough space is broken.
> 
> Jonathan
> 
> > ---
> >  .clang-format                                 |   1 +
> >  .../userspace-api/ioctl/ioctl-number.rst      |   1 +
> >  drivers/cxl/mem.c                             | 288 +++++++++++++++++-
> >  include/uapi/linux/cxl_mem.h                  | 154 ++++++++++
> >  4 files changed, 443 insertions(+), 1 deletion(-)
> >  create mode 100644 include/uapi/linux/cxl_mem.h
> > 
> > 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/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@...fin.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 410adb1bdffc..a4298cb1182d 100644
> > --- a/drivers/cxl/mem.c
> > +++ b/drivers/cxl/mem.c
> > @@ -1,5 +1,6 @@
> >  // SPDX-License-Identifier: GPL-2.0-only
> >  /* Copyright(c) 2020 Intel Corporation. All rights reserved. */
> > +#include <uapi/linux/cxl_mem.h>
> >  #include <linux/module.h>
> >  #include <linux/mutex.h>
> >  #include <linux/cdev.h>
> > @@ -40,6 +41,7 @@
> >  #define CXL_MAILBOX_TIMEOUT_MS (2 * HZ)
> >  
> >  enum opcode {
> > +	CXL_MBOX_OP_INVALID		= 0x0000,
> >  	CXL_MBOX_OP_IDENTIFY		= 0x4000,
> >  	CXL_MBOX_OP_MAX			= 0x10000
> >  };
> > @@ -91,6 +93,49 @@ struct cxl_memdev {
> >  static int cxl_mem_major;
> >  static DEFINE_IDA(cxl_memdev_ida);
> >  
> > +/**
> > + * 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
> > + *
> > + * 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 {
> > +	struct cxl_command_info info;
> > +	enum opcode opcode;
> > +};
> > +
> > +#define CXL_CMD(_id, sin, sout)                                                \
> > +	[CXL_MEM_COMMAND_ID_##_id] = {                                         \
> > +	.info =	{                                                              \
> > +			.id = CXL_MEM_COMMAND_ID_##_id,                        \
> > +			.size_in = sin,                                        \
> > +			.size_out = sout,                                      \
> > +		},                                                             \
> > +	.opcode = CXL_MBOX_OP_##_id,                                           \
> > +	}
> > +
> > +/*
> > + * 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(IDENTIFY, 0, 0x43),
> > +};
> > +
> > +#define cxl_for_each_cmd(cmd)                                                  \
> > +	for ((cmd) = &mem_commands[0];                                         \
> > +	     ((cmd) - mem_commands) < ARRAY_SIZE(mem_commands); (cmd)++)
> > +
> > +#define cxl_cmd_count ARRAY_SIZE(mem_commands)
> > +
> >  static int cxl_mem_wait_for_doorbell(struct cxl_mem *cxlm)
> >  {
> >  	const unsigned long start = jiffies;
> > @@ -312,6 +357,247 @@ static void cxl_mem_mbox_put(struct cxl_mem *cxlm)
> >  	mutex_unlock(&cxlm->mbox_mutex);
> >  }
> >  
> > +/**
> > + * handle_mailbox_cmd_from_user() - Dispatch a mailbox command for userspace.
> > + * @cxlm: The CXL memory device to communicate with.
> > + * @cmd: The validated command.
> > + * @in_payload: Pointer to userspace's input payload.
> > + * @out_payload: Pointer to userspace's output payload.
> > + * @size_out: (Input) Max payload size to copy out.
> > + *            (Output) Payload size hardware generated.
> > + * @retval: Hardware generated return code from the operation.
> > + *
> > + * Return:
> > + *  * %0	- Mailbox transaction succeeded. This implies the mailbox
> > + *  		  protocol completed successfully not that the operation itself
> > + *  		  was successful.
> > + *  * %-ENOMEM  - Couldn't allocate a bounce buffer.
> > + *  * %-EFAULT	- Something happened with copy_to/from_user.
> > + *  * %-EINTR	- Mailbox acquisition interrupted.
> > + *  * %-*	- Transaction level failures.
> > + *
> > + * Creates the appropriate mailbox command and dispatches it on behalf of a
> > + * userspace request. The input and output payloads are copied between
> > + * userspace.
> > + *
> > + * See cxl_send_cmd().
> > + */
> > +static int handle_mailbox_cmd_from_user(struct cxl_mem *cxlm,
> > +					const struct cxl_mem_command *cmd,
> > +					u64 in_payload, u64 out_payload,
> > +					s32 *size_out, u32 *retval)
> > +{
> > +	struct device *dev = &cxlm->pdev->dev;
> > +	struct mbox_cmd mbox_cmd = {
> > +		.opcode = cmd->opcode,
> > +		.size_in = cmd->info.size_in,
> > +	};
> > +	int rc;
> > +
> > +	if (cmd->info.size_out) {
> > +		mbox_cmd.payload_out = kvzalloc(cmd->info.size_out, GFP_KERNEL);
> > +		if (!mbox_cmd.payload_out)
> > +			return -ENOMEM;
> > +	}
> > +
> > +	if (cmd->info.size_in) {
> > +		mbox_cmd.payload_in = kvzalloc(cmd->info.size_in, GFP_KERNEL);
> > +		if (!mbox_cmd.payload_in) {
> > +			rc = -ENOMEM;
> > +			goto out;
> > +		}
> > +
> > +		if (copy_from_user(mbox_cmd.payload_in,
> > +				   u64_to_user_ptr(in_payload),
> > +				   cmd->info.size_in)) {
> > +			rc = -EFAULT;
> > +			goto out;
> > +		}
> > +	}
> > +
> > +	rc = cxl_mem_mbox_get(cxlm);
> > +	if (rc)
> > +		goto out;
> > +
> > +	dev_dbg(dev,
> > +		"Submitting %s command for user\n"
> > +		"\topcode: %x\n"
> > +		"\tsize: %ub\n",
> > +		cxl_command_names[cmd->info.id].name, mbox_cmd.opcode,
> > +		cmd->info.size_in);
> > +
> > +	rc = __cxl_mem_mbox_send_cmd(cxlm, &mbox_cmd);
> > +	cxl_mem_mbox_put(cxlm);
> > +	if (rc)
> > +		goto out;
> > +
> > +	/*
> > +	 * @size_out contains the max size that's allowed to be written back out
> > +	 * to userspace. While the payload may have written more output than
> > +	 * this it will have to be ignored.
> > +	 */
> 
> See below for why I don't think this works. The size of mbox_cmd.payload_out
> seems to always be the size userspace specified, never the 1MB this code
> is assuming.  So if the hardware returns more than userspace asks for you
> have a buffer overrun.
> 
> 
> > +	if (mbox_cmd.size_out) {
> > +		if (copy_to_user(u64_to_user_ptr(out_payload),
> > +				 mbox_cmd.payload_out, *size_out)) {
> > +			rc = -EFAULT;
> > +			goto out;
> > +		}
> > +	}
> > +
> > +	/*
> > +	 * Reporting the actual size, even if it was greater than @size_out
> > +	 * allows userspace to try the command again with a bigger buffer.
> > +	 */
> > +	*size_out = mbox_cmd.size_out;
> > +	*retval = mbox_cmd.return_code;
> > +
> > +out:
> > +	kvfree(mbox_cmd.payload_in);
> > +	kvfree(mbox_cmd.payload_out);
> > +	return rc;
> > +}
> > +
> > +/**
> > + * cxl_validate_cmd_from_user() - Check fields for CXL_MEM_SEND_COMMAND.
> > + * @cxlm: &struct cxl_mem device whose mailbox will be used.
> > + * @send_cmd: &struct cxl_send_command copied in from userspace.
> > + * @out_cmd: Sanitized and populated &struct cxl_mem_command.
> > + *
> > + * Return:
> > + *  * %0	- @out_cmd is ready to send.
> > + *  * %-ENOTTY	- Invalid command specified.
> > + *  * %-EINVAL	- Reserved fields or invalid values were used.
> > + *  * %-ENOMEM	- Input or output buffer wasn't sized properly.
> > + *
> > + * The result of this command is a fully validated command in @out_cmd that is
> > + * safe to send to the hardware.
> > + *
> > + * See handle_mailbox_cmd_from_user()
> > + */
> > +static int cxl_validate_cmd_from_user(struct cxl_mem *cxlm,
> > +				      const struct cxl_send_command *send_cmd,
> > +				      struct cxl_mem_command *out_cmd)
> > +{
> > +	const struct cxl_command_info *info;
> > +	struct cxl_mem_command *c;
> > +
> > +	if (send_cmd->id == 0 || send_cmd->id >= CXL_MEM_COMMAND_ID_MAX)
> > +		return -ENOTTY;
> > +
> > +	/*
> > +	 * The user can never specify an input payload larger than what hardware
> > +	 * supports, but output can be arbitrarily large (simply write out as
> > +	 * much data as the hardware provides).
> > +	 */
> > +	if (send_cmd->in.size > cxlm->payload_size)
> > +		return -EINVAL;
> > +
> > +	if (send_cmd->flags & ~CXL_MEM_COMMAND_FLAG_MASK)
> > +		return -EINVAL;
> > +
> > +	if (send_cmd->rsvd)
> > +		return -EINVAL;
> > +
> > +	if (send_cmd->in.rsvd || send_cmd->out.rsvd)
> > +		return -EINVAL;
> > +
> > +	/* Convert user's command into the internal representation */
> > +	c = &mem_commands[send_cmd->id];
> > +	info = &c->info;
> > +
> > +	/* Check the input buffer is the expected size */
> > +	if (info->size_in >= 0 && info->size_in != send_cmd->in.size)
> > +		return -ENOMEM;
> > +
> > +	/* Check the output buffer is at least large enough */
> > +	if (info->size_out >= 0 && send_cmd->out.size < info->size_out)
> > +		return -ENOMEM;
> > +
> > +	memcpy(out_cmd, c, sizeof(*c));
> > +	out_cmd->info.size_in = send_cmd->in.size;
> > +	out_cmd->info.size_out = send_cmd->out.size;
> > +
> > +	return 0;
> > +}
> > +
> > +static int cxl_query_cmd(struct cxl_memdev *cxlmd,
> > +			 struct cxl_mem_query_commands __user *q)
> > +{
> > +	struct device *dev = &cxlmd->dev;
> > +	struct cxl_mem_command *cmd;
> > +	u32 n_commands;
> > +	int j = 0;
> > +
> > +	dev_dbg(dev, "Query IOCTL\n");
> > +
> > +	if (get_user(n_commands, &q->n_commands))
> > +		return -EFAULT;
> > +
> > +	/* returns the total number if 0 elements are requested. */
> > +	if (n_commands == 0)
> > +		return put_user(cxl_cmd_count, &q->n_commands);
> > +
> > +	/*
> > +	 * otherwise, return max(n_commands, total commands) cxl_command_info
> > +	 * structures.
> > +	 */
> > +	cxl_for_each_cmd(cmd) {
> > +		const struct cxl_command_info *info = &cmd->info;
> > +
> > +		if (copy_to_user(&q->commands[j++], info, sizeof(*info)))
> > +			return -EFAULT;
> > +
> > +		if (j == n_commands)
> > +			break;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static int cxl_send_cmd(struct cxl_memdev *cxlmd,
> > +			struct cxl_send_command __user *s)
> > +{
> > +	struct cxl_mem *cxlm = cxlmd->cxlm;
> > +	struct device *dev = &cxlmd->dev;
> > +	struct cxl_send_command send;
> > +	struct cxl_mem_command c;
> > +	int rc;
> > +
> > +	dev_dbg(dev, "Send IOCTL\n");
> > +
> > +	if (copy_from_user(&send, s, sizeof(send)))
> > +		return -EFAULT;
> > +
> > +	rc = cxl_validate_cmd_from_user(cxlmd->cxlm, &send, &c);
> > +	if (rc)
> > +		return rc;
> 
> Userspace will pass in send.out set to the size of it's available buffer.
> Then cxl_validate_cmd_from_user() will fill
> c.info.size_out with send.out.size
> 
> > +
> > +	/* Prepare to handle a full payload for variable sized output */
> > +	if (c.info.size_out < 0)
> 
> So this check only works if userspace set the command to have variable size.
> That's not what the docs below suggest should happen.
> 

Another good catch. This bug was introduced after the last change was made. I
still don't want to have a size_out as part of the mailbox command.

Validate should not alter the size_out field except in the case of RAW commands.

I believe this is the right fix, handle_mailbox_cmd_from_user() already will
only copy_to the right number of byte, but your eyes on it are appreciated.

diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
index 237b956f0be0..4ca4f5afd9d2 100644
--- a/drivers/cxl/mem.c
+++ b/drivers/cxl/mem.c
@@ -686,7 +686,11 @@ static int cxl_validate_cmd_from_user(struct cxl_mem *cxlm,

        memcpy(out_cmd, c, sizeof(*c));
        out_cmd->info.size_in = send_cmd->in.size;
-       out_cmd->info.size_out = send_cmd->out.size;
+       /*
+        * XXX: out_cmd->info.size_out will be controlled by the driver, and the
+        * specified number of bytes @send_cmd->out.size will be copied back out
+        * to userspace.
+        */

        return 0;
 }

> > +		c.info.size_out = cxlm->payload_size;
> > +
> > +	rc = handle_mailbox_cmd_from_user(cxlm, &c, send.in.payload,
> > +					  send.out.payload, &send.out.size,
> > +					  &send.retval);
> > +	if (rc)
> > +		return rc;
> > +
> > +	return copy_to_user(s, &send, sizeof(send));
> > +}
> > +
> > +static long __cxl_memdev_ioctl(struct cxl_memdev *cxlmd, unsigned int cmd,
> > +			       unsigned long arg)
> > +{
> > +	switch (cmd) {
> > +	case CXL_MEM_QUERY_COMMANDS:
> > +		return cxl_query_cmd(cxlmd, (void __user *)arg);
> > +	case CXL_MEM_SEND_COMMAND:
> > +		return cxl_send_cmd(cxlmd, (void __user *)arg);
> > +	default:
> > +		return -ENOTTY;
> > +	}
> > +}
> > +
> >  static long cxl_memdev_ioctl(struct file *file, unsigned int cmd,
> >  			     unsigned long arg)
> >  {
> > @@ -325,7 +611,7 @@ static long cxl_memdev_ioctl(struct file *file, unsigned int cmd,
> >  	if (!percpu_ref_tryget_live(&cxlmd->ops_active))
> >  		return -ENXIO;
> >  
> > -	/* TODO: ioctl body */
> > +	rc = __cxl_memdev_ioctl(cxlmd, cmd, arg);
> >  
> >  	percpu_ref_put(&cxlmd->ops_active);
> >  
> > diff --git a/include/uapi/linux/cxl_mem.h b/include/uapi/linux/cxl_mem.h
> > new file mode 100644
> > index 000000000000..18cea908ad0b
> > --- /dev/null
> > +++ b/include/uapi/linux/cxl_mem.h
> > @@ -0,0 +1,154 @@
> > +/* 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_
> > +
> > +#include <linux/types.h>
> > +
> > +/**
> > + * DOC: UAPI
> > + *
> > + * 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)
> > +#define CXL_MEM_SEND_COMMAND _IOWR(0xCE, 2, struct cxl_send_command)
> > +
> > +#define CXL_CMDS                                                          \
> > +	___C(INVALID, "Invalid Command"),                                 \
> > +	___C(IDENTIFY, "Identify Command"),                               \
> > +	___C(MAX, "invalid / last command")
> > +
> > +#define ___C(a, b) CXL_MEM_COMMAND_ID_##a
> > +enum { CXL_CMDS };
> > +
> > +#undef ___C
> > +#define ___C(a, b) { b }
> > +static const struct {
> > +	const char *name;
> > +} cxl_command_names[] = { CXL_CMDS };
> > +
> > +/*
> > + * Here's how this actually breaks out:
> > + * cxl_command_names[] = {
> > + *	[CXL_MEM_COMMAND_ID_INVALID] = { "Invalid Command" },
> > + *	[CXL_MEM_COMMAND_ID_IDENTIFY] = { "Identify Command" },
> > + *	...
> > + *	[CXL_MEM_COMMAND_ID_MAX] = { "invalid / last command" },
> > + * };
> > + */
> 
> Thanks, this is great.
> 
> > +
> > +#undef ___C
> > +
> > +/**
> > + * struct cxl_command_info - Command information returned from a query.
> > + * @id: ID number for the command.
> > + * @flags: Flags that specify command behavior.
> > + * @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. This is returned as part of an array from the query ioctl. The
> > + * following would be a command that takes a variable length input and returns 0
> > + * bytes of output.
> > + *
> > + *  - @id = 10
> > + *  - @flags = 0
> > + *  - @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_MASK GENMASK(0, 0)
> > +
> > +	__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
> > +	 */
> 
> This is fairly well described in the docs above the structure.
> Perhaps combine the two.
> 
> > +	__u32 n_commands;
> > +	__u32 rsvd;
> > +
> > +	struct cxl_command_info __user commands[]; /* out: supported commands */
> > +};
> > +
> > +/**
> > + * struct cxl_send_command - Send a command to a memory device.
> > + * @id: The command to send to the memory device. This must be one of the
> > + *	commands returned by the query command.
> > + * @flags: Flags for the command (input).
> > + * @rsvd: Must be zero.
> > + * @retval: Return value from the memory device (output).
> > + * @in.size: Size of the payload to provide to the device (input).
> > + * @in.rsvd: Must be zero.
> > + * @in.payload: Pointer to memory for payload input, payload is little endian.
> > + * @out.size: Size of the payload received from the device (input/output). This
> > + *	      field is filled in by userspace to let the driver know how much
> > + *	      space was allocated for output. It is populated by the driver to
> > + *	      let userspace know how large the output payload actually was.
> > + * @out.rsvd: Must be zero.
> > + * @out.payload: Pointer to memory for payload output, payload is little endian.
> > + *
> > + * Mechanism for userspace to send a command to the hardware for processing. The
> > + * driver will do basic validation on the command sizes. In some cases even the
> > + * payload may be introspected. Userspace is required to allocate large enough
> > + * buffers for size_out which can be variable length in certain situations.
> > + */
> > +struct cxl_send_command {
> > +	__u32 id;
> > +	__u32 flags;
> > +	__u32 rsvd;
> > +	__u32 retval;
> > +
> > +	struct {
> > +		__s32 size;
> > +		__u32 rsvd;
> > +		__u64 payload;
> > +	} in;
> > +
> > +	struct {
> > +		__s32 size;
> > +		__u32 rsvd;
> > +		__u64 payload;
> > +	} out;
> > +};
> > +
> > +#endif
> 

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