linux-kernel - Re: [PATCH V4 3/5] misc: mlx5ctl: Add info ioctl

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Date: Sat, 2 Mar 2024 00:03:49 -0800
From: Saeed Mahameed <saeed@...nel.org>
To: Vegard Nossum <vegard.nossum@...cle.com>
Cc: Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-api@...r.kernel.org,
	Linux Documentation <linux-doc@...r.kernel.org>,
	Leon Romanovsky <leonro@...dia.com>,
	Jason Gunthorpe <jgg@...dia.com>, Jiri Pirko <jiri@...dia.com>,
	Leonid Bloch <lbloch@...dia.com>, Itay Avraham <itayavr@...dia.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Saeed Mahameed <saeedm@...dia.com>,
	David Ahern <dsahern@...nel.org>,
	Aron Silverton <aron.silverton@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	andrew.gospodarek@...adcom.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 3/5] misc: mlx5ctl: Add info ioctl

On 29 Feb 12:47, Vegard Nossum wrote:
>
>On 07/02/2024 08:24, Saeed Mahameed wrote:
>>+static int mlx5ctl_info_ioctl(struct file *file,
>>+			      struct mlx5ctl_info __user *arg,
>>+			      size_t usize)
>>+{
>>+	struct mlx5ctl_fd *mfd = file->private_data;
>>+	size_t ksize = sizeof(struct mlx5ctl_info);
>>+	struct mlx5ctl_dev *mcdev = mfd->mcdev;
>>+	struct mlx5_core_dev *mdev = mcdev->mdev;
>>+	struct mlx5ctl_info *info;
>>+	int err = 0;
>>+
>>+	if (usize < ksize)
>>+		return -EINVAL;
>>+
>>+	info = kzalloc(ksize, GFP_KERNEL);
>>+	if (!info)
>>+		return -ENOMEM;
>
>struct mlx5ctl_info is small, why not put it on the stack or even copy
>it directly from the original object, assuming it has no holes/padding?
>

There's no original object, but yes storing it on the stack should work.

>>+
>>+	info->dev_uctx_cap = MLX5_CAP_GEN(mdev, uctx_cap);
>>+	info->uctx_cap = mfd->uctx_cap;
>>+	info->uctx_uid = mfd->uctx_uid;
>>+	info->ucap = mfd->ucap;
>>+
>>+	if (copy_to_user(arg, info, ksize))
>>+		err = -EFAULT;
>>+
>>+	kfree(info);
>>+	return err;
>>+}
>
>Is there even a remote possibility of extending this structure in the
>future? If so the size check will not allow you to be backwards
>compatible. Should there be a version field in there or would you
>just add a new ioctl altogether? Adding linux-api@...r.kernel.org to Cc.
>

This was my original implementation, but Greg's preference is to allow no
extension to the ioctl structures, in case of extension required, new IOCTL
and structure should be introduced.

>>diff --git a/include/uapi/misc/mlx5ctl.h b/include/uapi/misc/mlx5ctl.h
>>new file mode 100644
>>index 000000000000..9be944128025
>>--- /dev/null
>>+++ b/include/uapi/misc/mlx5ctl.h
>>@@ -0,0 +1,20 @@
>>+/* SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0 WITH Linux-syscall-note */
>>+/* Copyright (c) 2023, NVIDIA CORPORATION & AFFILIATES. All rights reserved. */
>>+
>>+#ifndef __MLX5CTL_IOCTL_H__
>>+#define __MLX5CTL_IOCTL_H__
>>+
>>+struct mlx5ctl_info {
>>+	__u16 uctx_uid; /* current process allocated UCTX UID */
>>+	__u16 reserved1; /* explicit padding must be zero */
>>+	__u32 uctx_cap; /* current process effective UCTX cap */
>>+	__u32 dev_uctx_cap; /* device's UCTX capabilities */
>>+	__u32 ucap; /* process user capability */
>>+};
>>+
>>+#define MLX5CTL_IOCTL_MAGIC 0x5c
>>+
>>+#define MLX5CTL_IOCTL_INFO \
>>+	_IOR(MLX5CTL_IOCTL_MAGIC, 0x0, struct mlx5ctl_info)
>>+
>>+#endif /* __MLX5CTL_IOCTL_H__ */
>
>Should you add anything to Documentation/ABI/ ? (Or add other
>documentation for this driver?)
>

The driver doesn't expose any sysfs other than the IOCTLs, but yes
a documentation might be useful to make sure ABI is stable, most of the
other drivers point out to the uapi header for documentation.