Re: [PATCH vfio 2/6] vfio: Add support for VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2

[Alex Williamson <alex@shazbot.org>]

On Tue, 24 Feb 2026 10:20:15 +0200
Yishai Hadas <yishaih@nvidia.com> wrote:

> Currently, existing VFIO_MIG_GET_PRECOPY_INFO implementations don't
> assign info.flags before copy_to_user().
> 
> Because they copy the struct in from userspace first, this effectively
> echoes userspace-provided flags back as output, preventing the field
> from being used to report new reliable data from the drivers.
> 
> Add support for a new device feature named
> VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2.
> 
> On SET, enables the v2 pre_copy_info behaviour, where the
> vfio_precopy_info.flags is a valid output field.
> 
> Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
> ---
>  drivers/vfio/pci/vfio_pci_core.c |  1 +
>  drivers/vfio/vfio_main.c         | 20 ++++++++++++++++++++
>  include/linux/vfio.h             |  1 +
>  3 files changed, 22 insertions(+)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index d43745fe4c84..e22280f53ebf 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -736,6 +736,7 @@ void vfio_pci_core_close_device(struct vfio_device *core_vdev)
>  #endif
>  	vfio_pci_core_disable(vdev);
>  
> +	core_vdev->precopy_info_flags_fix = 0;
>  	vfio_pci_dma_buf_cleanup(vdev);
>  
>  	mutex_lock(&vdev->igate);
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 742477546b15..2243a6eb5547 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -964,6 +964,23 @@ vfio_ioctl_device_feature_migration_data_size(struct vfio_device *device,
>  	return 0;
>  }
>  
> +static int
> +vfio_ioctl_device_feature_migration_precopy_info_v2(struct vfio_device *device,
> +						    u32 flags, size_t argsz)
> +{
> +	int ret;
> +
> +	if (!(device->migration_flags & VFIO_MIGRATION_PRE_COPY))
> +		return -EINVAL;
> +
> +	ret = vfio_check_feature(flags, argsz, VFIO_DEVICE_FEATURE_SET, 0);

This should be VFIO_DEVICE_FEATURE_SET | VFIO_DEVICE_FEATURE_PROBE.
Probe support is essentially free, but we've not been good about
including it.  Thanks,

Alex

> +	if (ret != 1)
> +		return ret;
> +
> +	device->precopy_info_flags_fix = 1;
> +	return 0;
> +}
> +
>  static int vfio_ioctl_device_feature_migration(struct vfio_device *device,
>  					       u32 flags, void __user *arg,
>  					       size_t argsz)
> @@ -1251,6 +1268,9 @@ static int vfio_ioctl_device_feature(struct vfio_device *device,
>  		return vfio_ioctl_device_feature_migration_data_size(
>  			device, feature.flags, arg->data,
>  			feature.argsz - minsz);
> +	case VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2:
> +		return vfio_ioctl_device_feature_migration_precopy_info_v2(
> +			device, feature.flags, feature.argsz - minsz);
>  	default:
>  		if (unlikely(!device->ops->device_feature))
>  			return -ENOTTY;
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index e90859956514..3ff21374aeee 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -52,6 +52,7 @@ struct vfio_device {
>  	struct vfio_device_set *dev_set;
>  	struct list_head dev_set_list;
>  	unsigned int migration_flags;
> +	u8 precopy_info_flags_fix;
>  	struct kvm *kvm;
>  
>  	/* Members below here are private, not for driver use */