Re: [PATCH v5 3/5] iommufd/viommu: Keep a reference to the KVM file

[Souradeep Chakrabarti]

On Mon, May 25, 2026 at 09:18:14PM +0530, Aneesh Kumar K.V (Arm) wrote:
> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> 
> The TSM vDevice operations need access to the KVM associated with the
> device's vIOMMU. Save the device's KVM file in the iommufd_viommu when the
> vIOMMU is allocated, and take a file reference so it remains valid for the
> lifetime of the vIOMMU.
> 
> Release the reference when the vIOMMU is destroyed.
> 
> Based on an original patch by Shameer Kolothum <shameerali.kolothum.thodi@xxxxxxxxxx>
> 
> [nicolinc: hold kvm's users_count]
> Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> [aneesh.kumar: Switch to use kvm_file]
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@xxxxxxxxxx>
> ---
>  drivers/iommu/iommufd/viommu.c | 5 +++++
>  include/linux/iommufd.h        | 1 +
>  2 files changed, 6 insertions(+)
> 
> diff --git a/drivers/iommu/iommufd/viommu.c b/drivers/iommu/iommufd/viommu.c
> index 4081deda9b33..bf5d58d55939 100644
> --- a/drivers/iommu/iommufd/viommu.c
> +++ b/drivers/iommu/iommufd/viommu.c
> @@ -1,6 +1,7 @@
>  // SPDX-License-Identifier: GPL-2.0-only
>  /* Copyright (c) 2024, NVIDIA CORPORATION & AFFILIATES
>   */
> +#include <linux/file.h>
>  #include "iommufd_private.h"
>  
>  void iommufd_viommu_destroy(struct iommufd_object *obj)
> @@ -11,6 +12,8 @@ void iommufd_viommu_destroy(struct iommufd_object *obj)
>  	if (viommu->ops && viommu->ops->destroy)
>  		viommu->ops->destroy(viommu);
>  	refcount_dec(&viommu->hwpt->common.obj.users);
> +	if (viommu->kvm_file)
> +		fput(viommu->kvm_file);
>  	xa_destroy(&viommu->vdevs);
>  }
>  
> @@ -76,6 +79,8 @@ int iommufd_viommu_alloc_ioctl(struct iommufd_ucmd *ucmd)
>  	}
>  
>  	xa_init(&viommu->vdevs);
> +	if (idev->kvm_file)
> +		viommu->kvm_file = get_file(idev->kvm_file);
There is now no uapi path at all for a non-KVM VMM to associate its VM
with a viommu. Can we (re)introduce an explicit, neutral
association, that accepts a KVM fd or another VMM's VM fd, in addition
to the implicit VFIO path?
>  	viommu->type = cmd->type;
>  	viommu->ictx = ucmd->ictx;
>  	viommu->hwpt = hwpt_paging;
> diff --git a/include/linux/iommufd.h b/include/linux/iommufd.h
> index 0a0bb4abfbd2..3267717f676d 100644
> --- a/include/linux/iommufd.h
> +++ b/include/linux/iommufd.h
> @@ -103,6 +103,7 @@ struct iommufd_viommu {
>  	struct iommufd_ctx *ictx;
>  	struct iommu_device *iommu_dev;
>  	struct iommufd_hwpt_paging *hwpt;
> +	struct file *kvm_file;
>  
>  	const struct iommufd_viommu_ops *ops;
>  
> -- 
> 2.43.0
>