Re: [PATCH v2 1/3] iommufd/viommu: Allow associating a KVM VM fd with a vIOMMU

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Mar 09, 2026 at 04:47:02PM +0530, Aneesh Kumar K.V (Arm) wrote:
> Add optional KVM association to IOMMU_VIOMMU_ALLOC by introducing
> IOMMU_VIOMMU_KVM_FD and iommu_viommu_alloc::kvm_vm_fd.
> 
> When the flag is set, iommufd validates that kvm_vm_fd refers to a KVM
> VM file and stores a referenced struct file in the vIOMMU object, so
> later iommufd operations can safely resolve the owning VM.
> 
> This is preparatory plumbing for subsequent patches that bind TDI state
> to the associated KVM VM.
> 
> The patch also switch file_is_kvm from EXPORT_SYMBOL_FOR_KVM_INTERNAL to
> EXPORT_SYMBOL_GPL so that iommu module can use that.

I'd want Sean to approve this..

But given we once discussed it is better to hold on to the file * than
the kvm as it leaks less kvm stuff I think this is the right approach.

>  #include <linux/types.h>
>  #include <linux/xarray.h>
> +#include <linux/file.h>
>  #include <uapi/linux/iommufd.h>
>  
>  struct device;
> @@ -58,6 +59,7 @@ struct iommufd_object {
>  	unsigned int id;
>  };
>  
> +struct kvm;

That is a weird place to put the forward declaration? How about at the
top of the file with the others?

> +/**
> + * define IOMMU_VIOMMU_KVM_FD - Flag indicating a valid KVM VM file descriptor
> + *
> + * Set this flag when allocating a viommu instance that should be associated
> + * with a specific KVM VM. If this flag is not provided,
> + * @iommu_viommu_alloc::kvm_vm_fd is ignored.
> + */
> +#define IOMMU_VIOMMU_KVM_FD	BIT(0)

BIT isn't used in this uapi header, lets not start.

Other cases are using an enum to group the flags

>  /**
>   * struct iommu_viommu_alloc - ioctl(IOMMU_VIOMMU_ALLOC)
>   * @size: sizeof(struct iommu_viommu_alloc)
> - * @flags: Must be 0
> + * @flags: Supported flags (IOMMU_VIOMMU_KVM_FD)

So you can say 'see enum xyz' here

> @@ -1105,6 +1115,7 @@ struct iommu_viommu_alloc {
>  	__u32 data_len;
>  	__u32 __reserved;
>  	__aligned_u64 data_uptr;
> +	__s32 kvm_vm_fd;

Add padding to keep to 8 byte alignment


> @@ -5481,7 +5481,7 @@ bool file_is_kvm(struct file *file)
>  {
>  	return file && file->f_op == &kvm_vm_fops;
>  }
> -EXPORT_SYMBOL_FOR_KVM_INTERNAL(file_is_kvm);
> +EXPORT_SYMBOL_GPL(file_is_kvm);

Probably put this in its own patch 

Jason