Re: [PATCH v7 4/4] vfio-pci/zdev: Add VFIO FMB device features

[Alex Williamson <alex@shazbot.org>]

On Tue, 22 Sep 2026 15:51:41 -0400
Omar Elghoul <oelghoul@linux.ibm.com> wrote:

> Introduce new VFIO features for zPCI devices to provide FMB passthrough to
> userspace.
> 
> Allow the user to enable or disable the FMB using the SET-only feature
> VFIO_DEVICE_FEATURE_ZPCI_FMB_ENABLE. Likewise allow the user to read the
> latest FMB using the GET-only feature VFIO_DEVICE_FEATURE_ZPCI_FMB_READ
> in the case when the FMB is enabled.
> 
> Signed-off-by: Omar Elghoul <oelghoul@linux.ibm.com>
> ---
>  drivers/vfio/pci/vfio_pci_core.c |  4 +++
>  drivers/vfio/pci/vfio_pci_priv.h | 18 ++++++++++
>  drivers/vfio/pci/vfio_pci_zdev.c | 60 ++++++++++++++++++++++++++++++++
>  include/uapi/linux/vfio.h        | 29 +++++++++++++++
>  4 files changed, 111 insertions(+)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index 6757054e9d87..3c827a77725b 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -1627,6 +1627,10 @@ int vfio_pci_core_ioctl_feature(struct vfio_device *device, u32 flags,
>  		return vfio_pci_core_feature_dma_buf(vdev, flags, arg, argsz);
>  	case VFIO_DEVICE_FEATURE_ZPCI_ERROR:
>  		return vfio_pci_zdev_feature_err(device, flags, arg, argsz);
> +	case VFIO_DEVICE_FEATURE_ZPCI_FMB_ENABLE:
> +		return vfio_pci_zdev_feature_fmb_enable(vdev, flags, arg, argsz);
> +	case VFIO_DEVICE_FEATURE_ZPCI_FMB_READ:
> +		return vfio_pci_zdev_feature_fmb_read(vdev, flags, arg, argsz);
>  	default:
>  		return -ENOTTY;
>  	}
> diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
> index 4e7162234a2e..e04d7e9d0c30 100644
> --- a/drivers/vfio/pci/vfio_pci_priv.h
> +++ b/drivers/vfio/pci/vfio_pci_priv.h
> @@ -95,6 +95,10 @@ int vfio_pci_zdev_open_device(struct vfio_pci_core_device *vdev);
>  void vfio_pci_zdev_close_device(struct vfio_pci_core_device *vdev);
>  int vfio_pci_zdev_feature_err(struct vfio_device *device, u32 flags,
>  			      void __user *arg, size_t argsz);
> +int vfio_pci_zdev_feature_fmb_enable(struct vfio_pci_core_device *vdev, u32 flags,
> +				     void __user *arg, size_t argsz);
> +int vfio_pci_zdev_feature_fmb_read(struct vfio_pci_core_device *vdev, u32 flags,
> +				   void __user *arg, size_t argsz);
>  #else
>  static inline int vfio_pci_info_zdev_add_caps(struct vfio_pci_core_device *vdev,
>  					      struct vfio_info_cap *caps)
> @@ -116,6 +120,20 @@ static inline int vfio_pci_zdev_feature_err(struct vfio_device *device,
>  {
>  	return -ENOTTY;
>  }
> +
> +static inline int vfio_pci_zdev_feature_fmb_enable(struct vfio_pci_core_device *vdev,
> +						   u32 flags, void __user *arg,
> +						   size_t argsz)
> +{
> +	return -ENOTTY;
> +}
> +
> +static inline int vfio_pci_zdev_feature_fmb_read(struct vfio_pci_core_device *vdev,
> +						 u32 flags, void __user *arg,
> +						 size_t argsz)
> +{
> +	return -ENOTTY;
> +}
>  #endif
>  
>  static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
> diff --git a/drivers/vfio/pci/vfio_pci_zdev.c b/drivers/vfio/pci/vfio_pci_zdev.c
> index f47f36314a1c..b028de8e08a4 100644
> --- a/drivers/vfio/pci/vfio_pci_zdev.c
> +++ b/drivers/vfio/pci/vfio_pci_zdev.c
> @@ -219,3 +219,63 @@ void vfio_pci_zdev_close_device(struct vfio_pci_core_device *vdev)
>  	if (zpci_kvm_hook.kvm_unregister)
>  		zpci_kvm_hook.kvm_unregister(zdev);
>  }
> +
> +int vfio_pci_zdev_feature_fmb_enable(struct vfio_pci_core_device *vdev, u32 flags,
> +				     void __user *arg, size_t argsz)
> +{
> +	struct zpci_dev *zdev;
> +	struct vfio_device_feature_zpci_fmb_enable fmb_enable;
> +	int ret;
> +
> +	ret = vfio_check_feature(flags, argsz, VFIO_DEVICE_FEATURE_SET, sizeof(fmb_enable));
> +	if (ret != 1)
> +		return ret;
> +
> +	zdev = to_zpci(vdev->pdev);
> +	if (!zdev)
> +		return -ENODEV;
> +
> +	if (copy_from_user(&fmb_enable, arg, sizeof(fmb_enable)))
> +		return -EFAULT;
> +
> +	guard(mutex)(&zdev->fmb_lock);
> +
> +	if (fmb_enable.enabled)
> +		return zpci_fmb_reenable_device(zdev);
> +	return zpci_fmb_disable_device(zdev);
> +}
> +
> +int vfio_pci_zdev_feature_fmb_read(struct vfio_pci_core_device *vdev, u32 flags,
> +				   void __user *arg, size_t argsz)
> +{
> +	struct zpci_dev *zdev;
> +	struct vfio_device_feature_zpci_fmb_read fmb_read;
> +	struct zpci_fmb fmb_bounce;
> +	int ret;
> +
> +	ret = vfio_check_feature(flags, argsz, VFIO_DEVICE_FEATURE_GET, sizeof(fmb_read));
> +	if (ret != 1)
> +		return ret;
> +
> +	zdev = to_zpci(vdev->pdev);
> +	if (!zdev)
> +		return -ENODEV;
> +
> +	if (copy_from_user(&fmb_read, arg, sizeof(fmb_read)))
> +		return -EFAULT;
> +	if (!fmb_read.data)
> +		return -EINVAL;
> +
> +	mutex_lock(&zdev->fmb_lock);
> +	if (!zdev->fmb_enabled) {
> +		mutex_unlock(&zdev->fmb_lock);
> +		return -ENOMSG;
> +	}
> +
> +	memcpy(&fmb_bounce, zdev->fmb, zdev->fmb_length);
> +	mutex_unlock(&zdev->fmb_lock);

A scoped guard would be preferable for consistency with the enable
path.

> +
> +	if (copy_to_user(u64_to_user_ptr(fmb_read.data), &fmb_bounce, zdev->fmb_length))
> +		return -EFAULT;
> +	return 0;
> +}
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index e41437fa17ad..a8d501e42ef9 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -1555,6 +1555,35 @@ struct vfio_device_feature_zpci_err {
>  
>  #define VFIO_DEVICE_FEATURE_ZPCI_ERROR 13
>  
> +/**
> + * Upon VFIO_DEVICE_FEATURE_SET, enable or disable FMB for the VFIO zPCI device.
> + *
> + * enabled is treated as a bool, so any non-zero value evaluates to true. Nested
> + * enabling resets the FMB counters to zero, and nested disabling fails.
> + *
> + * Returns: 0 on success, -1 and errno set appropriately on error.
> + */
> +#define VFIO_DEVICE_FEATURE_ZPCI_FMB_ENABLE 14
> +
> +struct vfio_device_feature_zpci_fmb_enable {
> +	__u8 enabled;
> +};
> +
> +/**
> + * Upon VFIO_DEVICE_FEATURE_GET, provide FMB passthrough for VFIO zPCI devices.
> + *
> + * The user-provided buffer must be at least fmb_length large, where fmb_length
> + * is reported in VFIO_DEVICE_INFO_CAP_ZPCI_BASE.
> + *
> + * Returns: 0 on success, -1 and errno set appropriately on error. errno==ENOMSG
> + * when the FMB is not enabled.
> + */
> +#define VFIO_DEVICE_FEATURE_ZPCI_FMB_READ 15
> +
> +struct vfio_device_feature_zpci_fmb_read {
> +	__aligned_u64 data;
> +};
> +

As usual, feature numbers need to be coordinated with the vfio tree.
What's the merge plan for this?  Thanks,

Alex