Re: [PATCH v4 4/9] cxl/mem: Add basic IOCTL interface
From: Ben Widawsky
Date: Tue Feb 16 2021 - 13:35:43 EST
On 21-02-16 18:28:49, Jonathan Cameron wrote:
> On Tue, 16 Feb 2021 09:53:14 -0800
> Ben Widawsky <ben.widawsky@xxxxxxxxx> wrote:
>
> > On 21-02-16 15:22:23, Jonathan Cameron wrote:
> > > On Mon, 15 Feb 2021 17:45:33 -0800
> > > Ben Widawsky <ben.widawsky@xxxxxxxxx> 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@xxxxxxxxx> # bug in earlier revision
> > > > Signed-off-by: Ben Widawsky <ben.widawsky@xxxxxxxxx>
> > > > Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx> (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@xxxxxxxxxxxxxxxxxxxx>
> > > > 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
I'm working on hardening __cxl_mem_mbox_send_cmd now per your request. With
that, I think this solves the issue, right?
>
>
> >
> > > > + 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
> > >
>