linux-kernel - Re: [PATCH 2/2 v4] vfio-cdx: add ioctl support for bus master enable

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Date:   Wed, 26 Jul 2023 11:03:22 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Nipun Gupta <nipun.gupta@....com>
Cc:     <gregkh@...uxfoundation.org>, <linux-kernel@...r.kernel.org>,
        <git@....com>, <pieter.jansen-van-vuuren@....com>,
        <nikhil.agarwal@....com>, <michal.simek@....com>,
        <abhijit.gangurde@....com>, Shubham Rohila <shubham.rohila@....com>
Subject: Re: [PATCH 2/2 v4] vfio-cdx: add ioctl support for bus master
 enable

On Wed, 26 Jul 2023 16:32:20 +0530
Nipun Gupta <nipun.gupta@....com> wrote:

> add VFIO_DEVICE_CDX_CTRL IOCTL to expose control operations for CDX
> devices to VFIO users. Support Bus master enable and Bus master disable
> on CDX bus control.
> 
> Signed-off-by: Shubham Rohila <shubham.rohila@....com>
> Signed-off-by: Nipun Gupta <nipun.gupta@....com>
> ---
> 
> Changes in v4:
> - This patch is newly added which uses cdx_set_master() and
>   cdx_clear_master() APIs.
> 
>  drivers/vfio/cdx/main.c   | 26 ++++++++++++++++++++++++++
>  include/uapi/linux/vfio.h | 14 ++++++++++++++
>  2 files changed, 40 insertions(+)
> 
> diff --git a/drivers/vfio/cdx/main.c b/drivers/vfio/cdx/main.c
> index c376a69d2db2..c39a965716f4 100644
> --- a/drivers/vfio/cdx/main.c
> +++ b/drivers/vfio/cdx/main.c
> @@ -98,6 +98,30 @@ static int vfio_cdx_ioctl_get_region_info(struct vfio_cdx_device *vdev,
>  	return copy_to_user(arg, &info, minsz) ? -EFAULT : 0;
>  }
>  
> +static int vfio_cdx_ioctl_ctrl(struct vfio_cdx_device *vdev,
> +			       struct vfio_device_cdx_ctrl __user *arg)
> +{
> +	unsigned long minsz = offsetofend(struct vfio_device_cdx_ctrl, op);
> +	struct cdx_device *cdx_dev = to_cdx_device(vdev->vdev.dev);
> +	struct vfio_device_cdx_ctrl ops;
> +
> +	if (copy_from_user(&ops, arg, minsz))
> +		return -EFAULT;
> +
> +	if (ops.argsz < minsz)
> +		return -EINVAL;
> +
> +	switch (ops.op) {
> +	case VFIO_CDX_CTRL_CLEAR_MASTER:
> +		cdx_clear_master(cdx_dev);
> +		return 0;
> +	case VFIO_CDX_CTRL_SET_MASTER:
> +		return cdx_set_master(cdx_dev);
> +	default:
> +		return -EINVAL;
> +	}
> +}
> +
>  static long vfio_cdx_ioctl(struct vfio_device *core_vdev,
>  			   unsigned int cmd, unsigned long arg)
>  {
> @@ -112,6 +136,8 @@ static long vfio_cdx_ioctl(struct vfio_device *core_vdev,
>  		return vfio_cdx_ioctl_get_region_info(vdev, uarg);
>  	case VFIO_DEVICE_RESET:
>  		return cdx_dev_reset(core_vdev->dev);
> +	case VFIO_DEVICE_CDX_CTRL:
> +		return vfio_cdx_ioctl_ctrl(vdev, uarg);
>  	default:
>  		return -ENOTTY;
>  	}
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index 20c804bdc09c..5f6a58f9f8e2 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -1649,6 +1649,20 @@ struct vfio_iommu_spapr_tce_remove {
>  };
>  #define VFIO_IOMMU_SPAPR_TCE_REMOVE	_IO(VFIO_TYPE, VFIO_BASE + 20)
>  
> +/**
> + * VFIO_DEVICE_CDX_CTRL - _IO(VFIO_TYPE, VFIO_BASE + 21)
> + *
> + * Control CDX device.
> + * Variable op is set as per the operation required
> + */
> +struct vfio_device_cdx_ctrl {
> +	__u32 argsz;
> +	__u32 op;
> +#define	VFIO_CDX_CTRL_SET_MASTER	0	/* Set Bus Master */
> +#define	VFIO_CDX_CTRL_CLEAR_MASTER	1	/* Clear Bus Master */
> +};
> +#define VFIO_DEVICE_CDX_CTRL		_IO(VFIO_TYPE, VFIO_BASE + 21)
> +
>  /* ***************************************************************** */
>  
>  #endif /* _UAPIVFIO_H */

This doesn't follow standard conventions for a vfio ioctl.  Not knowing
how CDX works, is there really a need for this ioctl vs enabling bus
master when the device is opened and clearing it when closed?  I think
this is effectively the behavior of vfio-platform.  Otherwise, I assume
this is not a high frequency operation, would the VFIO_DEVICE_FEATURE
ioctl be an option?  Thanks,

Alex