Re: [PATCH V4 04/10] iommufd: Add an ioctl IOMMU_IOAS_GET_PA to query PA from IOVA

[Alex Williamson <alex@shazbot.org>]

On Tue, 14 Apr 2026 14:14:06 -0700
Jacob Pan <jacob.pan@linux.microsoft.com> wrote:

> To support no-IOMMU mode where userspace drivers perform unsafe DMA
> using physical addresses, introduce a new API to retrieve the
> physical address of a user-allocated DMA buffer that has been mapped to
> an IOVA via IOAS. The mapping is backed by mock I/O page tables maintained
> by generic IOMMUPT framework.
> 
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
> v4:
>    - Fix unaligned IOVA length (Sashiko)
> v2:
>    - Scan the contiguous physical-address span beyond the first page and return its length.
> ---
>  drivers/iommu/iommufd/io_pagetable.c    | 60 +++++++++++++++++++++++++
>  drivers/iommu/iommufd/ioas.c            | 25 +++++++++++
>  drivers/iommu/iommufd/iommufd_private.h |  3 ++
>  drivers/iommu/iommufd/main.c            |  3 ++
>  include/uapi/linux/iommufd.h            | 25 +++++++++++
>  5 files changed, 116 insertions(+)
> 
> diff --git a/drivers/iommu/iommufd/io_pagetable.c b/drivers/iommu/iommufd/io_pagetable.c
> index ee003bb2f647..04336a8e12f5 100644
> --- a/drivers/iommu/iommufd/io_pagetable.c
> +++ b/drivers/iommu/iommufd/io_pagetable.c
> @@ -849,6 +849,66 @@ int iopt_unmap_iova(struct io_pagetable *iopt, unsigned long iova,
>  	return iopt_unmap_iova_range(iopt, iova, iova_last, unmapped);
>  }
>  
> +int iopt_get_phys(struct io_pagetable *iopt, unsigned long iova, u64 *paddr,
> +		  u64 *length)
> +{
> +	struct iopt_area *area;
> +	u64 tmp_length = 0;
> +	u64 tmp_paddr = 0;
> +	int rc = 0;
> +
> +	if (!IS_ENABLED(CONFIG_VFIO_NOIOMMU))
> +		return -EOPNOTSUPP;
> +
> +	down_read(&iopt->iova_rwsem);
> +	area = iopt_area_iter_first(iopt, iova, iova);
> +	if (!area || !area->pages) {
> +		rc = -ENOENT;
> +		goto unlock_exit;
> +	}
> +
> +	if (!area->storage_domain ||
> +	    area->storage_domain->owner != &iommufd_noiommu_ops) {
> +		rc = -EOPNOTSUPP;
> +		goto unlock_exit;
> +	}
> +
> +	*paddr = iommu_iova_to_phys(area->storage_domain, iova);
> +	if (!*paddr) {
> +		rc = -EINVAL;
> +		goto unlock_exit;
> +	}
> +
> +	tmp_length = PAGE_SIZE - offset_in_page(iova);
> +	tmp_paddr = *paddr;
> +	/*
> +	 * Scan the domain for the contiguous physical address length so that
> +	 * userspace search can be optimized for fewer ioctls.
> +	 */
> +	while (iova < iopt_area_last_iova(area)) {
> +		unsigned long next_iova;
> +		u64 next_paddr;
> +
> +		if (check_add_overflow(iova, PAGE_SIZE, &next_iova))
> +			break;
> +
> +		next_paddr = iommu_iova_to_phys(area->storage_domain, next_iova);
> +
> +		if (!next_paddr || next_paddr != tmp_paddr + PAGE_SIZE)
> +			break;
> +
> +		iova = next_iova;
> +		tmp_paddr += PAGE_SIZE;
> +		tmp_length += PAGE_SIZE;
> +	}
> +	*length = tmp_length;

If next_iova exceeds iopt_area_last_iova(area) AND exists in
storage_domain AND happens to be physically contiguous, tmp_length is
advanced and the return length exceeds the mapping.  Otherwise this
also always does an iommu_iova_to_phys() one iteration beyond the area
last iova.  Thanks,

Alex

> +
> +unlock_exit:
> +	up_read(&iopt->iova_rwsem);
> +
> +	return rc;
> +}
> +
>  int iopt_unmap_all(struct io_pagetable *iopt, unsigned long *unmapped)
>  {
>  	/* If the IOVAs are empty then unmap all succeeds */
> diff --git a/drivers/iommu/iommufd/ioas.c b/drivers/iommu/iommufd/ioas.c
> index fed06c2b728e..93cebb4c23bd 100644
> --- a/drivers/iommu/iommufd/ioas.c
> +++ b/drivers/iommu/iommufd/ioas.c
> @@ -375,6 +375,31 @@ int iommufd_ioas_unmap(struct iommufd_ucmd *ucmd)
>  	return rc;
>  }
>  
> +int iommufd_ioas_get_pa(struct iommufd_ucmd *ucmd)
> +{
> +	struct iommu_ioas_get_pa *cmd = ucmd->cmd;
> +	struct iommufd_ioas *ioas;
> +	int rc;
> +
> +	if (cmd->flags || cmd->__reserved)
> +		return -EOPNOTSUPP;
> +
> +	ioas = iommufd_get_ioas(ucmd->ictx, cmd->ioas_id);
> +	if (IS_ERR(ioas))
> +		return PTR_ERR(ioas);
> +
> +	rc = iopt_get_phys(&ioas->iopt, cmd->iova, &cmd->out_phys,
> +			   &cmd->out_length);
> +	if (rc)
> +		goto out_put;
> +
> +	rc = iommufd_ucmd_respond(ucmd, sizeof(*cmd));
> +out_put:
> +	iommufd_put_object(ucmd->ictx, &ioas->obj);
> +
> +	return rc;
> +}
> +
>  static void iommufd_release_all_iova_rwsem(struct iommufd_ctx *ictx,
>  					   struct xarray *ioas_list)
>  {
> diff --git a/drivers/iommu/iommufd/iommufd_private.h b/drivers/iommu/iommufd/iommufd_private.h
> index 2682b5baa6e9..0e772882aee9 100644
> --- a/drivers/iommu/iommufd/iommufd_private.h
> +++ b/drivers/iommu/iommufd/iommufd_private.h
> @@ -118,6 +118,8 @@ int iopt_map_pages(struct io_pagetable *iopt, struct list_head *pages_list,
>  int iopt_unmap_iova(struct io_pagetable *iopt, unsigned long iova,
>  		    unsigned long length, unsigned long *unmapped);
>  int iopt_unmap_all(struct io_pagetable *iopt, unsigned long *unmapped);
> +int iopt_get_phys(struct io_pagetable *iopt, unsigned long iova, u64 *paddr,
> +		  u64 *length);
>  
>  int iopt_read_and_clear_dirty_data(struct io_pagetable *iopt,
>  				   struct iommu_domain *domain,
> @@ -346,6 +348,7 @@ int iommufd_ioas_map_file(struct iommufd_ucmd *ucmd);
>  int iommufd_ioas_change_process(struct iommufd_ucmd *ucmd);
>  int iommufd_ioas_copy(struct iommufd_ucmd *ucmd);
>  int iommufd_ioas_unmap(struct iommufd_ucmd *ucmd);
> +int iommufd_ioas_get_pa(struct iommufd_ucmd *ucmd);
>  int iommufd_ioas_option(struct iommufd_ucmd *ucmd);
>  int iommufd_option_rlimit_mode(struct iommu_option *cmd,
>  			       struct iommufd_ctx *ictx);
> diff --git a/drivers/iommu/iommufd/main.c b/drivers/iommu/iommufd/main.c
> index 8c6d43601afb..ebae01ed947d 100644
> --- a/drivers/iommu/iommufd/main.c
> +++ b/drivers/iommu/iommufd/main.c
> @@ -432,6 +432,7 @@ union ucmd_buffer {
>  	struct iommu_veventq_alloc veventq;
>  	struct iommu_vfio_ioas vfio_ioas;
>  	struct iommu_viommu_alloc viommu;
> +	struct iommu_ioas_get_pa get_pa;
>  #ifdef CONFIG_IOMMUFD_TEST
>  	struct iommu_test_cmd test;
>  #endif
> @@ -484,6 +485,8 @@ static const struct iommufd_ioctl_op iommufd_ioctl_ops[] = {
>  		 struct iommu_ioas_map_file, iova),
>  	IOCTL_OP(IOMMU_IOAS_UNMAP, iommufd_ioas_unmap, struct iommu_ioas_unmap,
>  		 length),
> +	IOCTL_OP(IOMMU_IOAS_GET_PA, iommufd_ioas_get_pa, struct iommu_ioas_get_pa,
> +		 out_phys),
>  	IOCTL_OP(IOMMU_OPTION, iommufd_option, struct iommu_option, val64),
>  	IOCTL_OP(IOMMU_VDEVICE_ALLOC, iommufd_vdevice_alloc_ioctl,
>  		 struct iommu_vdevice_alloc, virt_id),
> diff --git a/include/uapi/linux/iommufd.h b/include/uapi/linux/iommufd.h
> index 1dafbc552d37..9afe0a1b11a0 100644
> --- a/include/uapi/linux/iommufd.h
> +++ b/include/uapi/linux/iommufd.h
> @@ -57,6 +57,7 @@ enum {
>  	IOMMUFD_CMD_IOAS_CHANGE_PROCESS = 0x92,
>  	IOMMUFD_CMD_VEVENTQ_ALLOC = 0x93,
>  	IOMMUFD_CMD_HW_QUEUE_ALLOC = 0x94,
> +	IOMMUFD_CMD_IOAS_GET_PA = 0x95,
>  };
>  
>  /**
> @@ -219,6 +220,30 @@ struct iommu_ioas_map {
>  };
>  #define IOMMU_IOAS_MAP _IO(IOMMUFD_TYPE, IOMMUFD_CMD_IOAS_MAP)
>  
> +/**
> + * struct iommu_ioas_get_pa - ioctl(IOMMU_IOAS_GET_PA)
> + * @size: sizeof(struct iommu_ioas_get_pa)
> + * @flags: Reserved, must be 0 for now
> + * @ioas_id: IOAS ID to query IOVA to PA mapping from
> + * @__reserved: Must be 0
> + * @iova: IOVA to query
> + * @out_length: Number of bytes contiguous physical address starting from phys
> + * @out_phys: Output physical address the IOVA maps to
> + *
> + * Query the physical address backing an IOVA range. The entire range must be
> + * mapped already. For noiommu devices doing unsafe DMA only.
> + */
> +struct iommu_ioas_get_pa {
> +	__u32 size;
> +	__u32 flags;
> +	__u32 ioas_id;
> +	__u32 __reserved;
> +	__aligned_u64 iova;
> +	__aligned_u64 out_length;
> +	__aligned_u64 out_phys;
> +};
> +#define IOMMU_IOAS_GET_PA _IO(IOMMUFD_TYPE, IOMMUFD_CMD_IOAS_GET_PA)
> +
>  /**
>   * struct iommu_ioas_map_file - ioctl(IOMMU_IOAS_MAP_FILE)
>   * @size: sizeof(struct iommu_ioas_map_file)