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Message-ID: <20210216182849.00002c8c@Huawei.com>
Date: Tue, 16 Feb 2021 18:28:49 +0000
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Ben Widawsky <ben.widawsky@...el.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 Tue, 16 Feb 2021 09:53:14 -0800
Ben Widawsky <ben.widawsky@...el.com> wrote:
> 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;
> }
This deals with the buffer overflow being triggered from userspace.
I'm still nervous. I really don't like assuming hardware will do the right
thing and never send us more data than we expect.
Given the check that it will fit in the target buffer is simple,
I'd prefer to harden it and know we can't have a problem.
Jonathan
>
> > > + 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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