Re: [PATCH v5 4/9] iommufd: Allow binding to a noiommu device

[Baolu Lu <baolu.lu@linux.intel.com>]

On 5/12/26 02:41, Jacob Pan wrote:
> From: Jason Gunthorpe <jgg@nvidia.com>
> 
> Allow iommufd to bind devices without an IOMMU (noiommu mode) by creating
> a dummy IOMMU group for such devices and skipping hwpt operations.
> 
> This enables noiommu devices to operate through the same iommufd API as IOMMU-
> capable devices.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
> ---
> v5:
>     - simplify logic and rename iommufd_device_is_noiommu (Kevin, Yi)
>     - use a helper iommufd_bind_noiommu instead of open coding (Kevin)
>     - move IOMMU cap check under iommufd_bind_iommu() (Yi)
>     - reword comments for partial init (Yi)
>     - misc minor clean up
> v4:
>     - Update the description of the module parameter (Alex)
> v3:
>     - Consolidate into fewer patches
> ---
>   drivers/iommu/iommufd/device.c | 148 ++++++++++++++++++++++++---------
>   1 file changed, 109 insertions(+), 39 deletions(-)
> 
> diff --git a/drivers/iommu/iommufd/device.c b/drivers/iommu/iommufd/device.c
> index d03076fcf3c2..4d75720432cc 100644
> --- a/drivers/iommu/iommufd/device.c
> +++ b/drivers/iommu/iommufd/device.c
> @@ -23,6 +23,16 @@ struct iommufd_attach {
>   	struct xarray device_array;
>   };
>   
> +/*
> + * A noiommu device has no IOMMU driver attached regardless of whether it
> + * enters via the cdev path (no iommu_group) or the group path (fake
> + * noiommu iommu_group). In both cases dev->iommu is NULL.
> + */
> +static bool iommufd_device_is_noiommu(struct iommufd_device *idev)
> +{
> +	return IS_ENABLED(CONFIG_IOMMUFD_NOIOMMU) && !idev->dev->iommu;

How about using device_iommu_mapped()? As I understand it, this is the
standard way to determine whether a device is protected by an IOMMU.

> +}
> +
>   static void iommufd_group_release(struct kref *kref)
>   {
>   	struct iommufd_group *igroup =
> @@ -30,9 +40,11 @@ static void iommufd_group_release(struct kref *kref)
>   
>   	WARN_ON(!xa_empty(&igroup->pasid_attach));
>   
> -	xa_cmpxchg(&igroup->ictx->groups, iommu_group_id(igroup->group), igroup,
> -		   NULL, GFP_KERNEL);
> -	iommu_group_put(igroup->group);
> +	if (igroup->group) {
> +		xa_cmpxchg(&igroup->ictx->groups, iommu_group_id(igroup->group),
> +			   igroup, NULL, GFP_KERNEL);
> +		iommu_group_put(igroup->group);
> +	}
>   	mutex_destroy(&igroup->lock);
>   	kfree(igroup);
>   }
> @@ -204,32 +216,19 @@ void iommufd_device_destroy(struct iommufd_object *obj)
>   	struct iommufd_device *idev =
>   		container_of(obj, struct iommufd_device, obj);
>   
> -	iommu_device_release_dma_owner(idev->dev);
> +	if (!idev->igroup)
> +		return;

I don't quite follow this logic. Is this check being added specifically
for the new noiommu mode? Since the noiommu mode introduces the
convention that "igroup->group == NULL" implies noiommu, there should be
no cases where idev->igroup itself is NULL.

> +	if (!iommufd_device_is_noiommu(idev))
> +		iommu_device_release_dma_owner(idev->dev);
>   	iommufd_put_group(idev->igroup);
>   	if (!iommufd_selftest_is_mock_dev(idev->dev))
>   		iommufd_ctx_put(idev->ictx);
>   }
>   
> -/**
> - * iommufd_device_bind - Bind a physical device to an iommu fd
> - * @ictx: iommufd file descriptor
> - * @dev: Pointer to a physical device struct
> - * @id: Output ID number to return to userspace for this device
> - *
> - * A successful bind establishes an ownership over the device and returns
> - * struct iommufd_device pointer, otherwise returns error pointer.
> - *
> - * A driver using this API must set driver_managed_dma and must not touch
> - * the device until this routine succeeds and establishes ownership.
> - *
> - * Binding a PCI device places the entire RID under iommufd control.
> - *
> - * The caller must undo this with iommufd_device_unbind()
> - */
> -struct iommufd_device *iommufd_device_bind(struct iommufd_ctx *ictx,
> -					   struct device *dev, u32 *id)
> +static int iommufd_bind_iommu(struct iommufd_device *idev)
>   {
> -	struct iommufd_device *idev;
> +	struct iommufd_ctx *ictx = idev->ictx;
> +	struct device *dev = idev->dev;
>   	struct iommufd_group *igroup;
>   	int rc;
>  

Thanks,
baolu