Re: [PATCH v2 13/22] iommufd: Add mmap interface

Baolu Lu
2025-04-28T02:57:04Z

On 4/26/25 13:58, Nicolin Chen wrote:
> For vIOMMU passing through HW resources to user space (VMs), add an mmap
> infrastructure to map a region of hardware MMIO pages.
>
> Maintain an mt_mmap per ictx for validations. To allow IOMMU drivers to
> add and delete mmappable regions to/from the mt_mmap, add a pair of new
> helpers: iommufd_ctx_alloc_mmap() and iommufd_ctx_free_mmap().

I am wondering why the dma_buf mechanism isn't used here, considering
that this also involves an export and import pattern.

>
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> ---
> drivers/iommu/iommufd/iommufd_private.h | 8 +++++
> include/linux/iommufd.h | 15 ++++++++++
> drivers/iommu/iommufd/driver.c | 39 +++++++++++++++++++++++++
> drivers/iommu/iommufd/main.c | 39 +++++++++++++++++++++++++
> 4 files changed, 101 insertions(+)
>
> diff --git a/drivers/iommu/iommufd/iommufd_private.h b/drivers/iommu/iommufd/iommufd_private.h
> index b974c207ae8a..db5b62ec4abb 100644
> --- a/drivers/iommu/iommufd/iommufd_private.h
> +++ b/drivers/iommu/iommufd/iommufd_private.h
> @@ -7,6 +7,7 @@
> #include <linux/iommu.h>
> #include <linux/iommufd.h>
> #include <linux/iova_bitmap.h>
> +#include <linux/maple_tree.h>
> #include <linux/rwsem.h>
> #include <linux/uaccess.h>
> #include <linux/xarray.h>
> @@ -44,6 +45,7 @@ struct iommufd_ctx {
> struct xarray groups;
> wait_queue_head_t destroy_wait;
> struct rw_semaphore ioas_creation_lock;
> + struct maple_tree mt_mmap;
>
> struct mutex sw_msi_lock;
> struct list_head sw_msi_list;
> @@ -55,6 +57,12 @@ struct iommufd_ctx {
> struct iommufd_ioas *vfio_ioas;
> };
>
> +/* Entry for iommufd_ctx::mt_mmap */
> +struct iommufd_mmap {
> + unsigned long pfn_start;
> + unsigned long pfn_end;
> +};

This structure is introduced to represent a mappable/mapped region,
right? It would be better to add comments specifying whether the start
and end are inclusive or exclusive.

> +
> /*
> * The IOVA to PFN map. The map automatically copies the PFNs into multiple
> * domains and permits sharing of PFNs between io_pagetable instances. This
> diff --git a/include/linux/iommufd.h b/include/linux/iommufd.h
> index 5dff154e8ce1..d63e2d91be0d 100644
> --- a/include/linux/iommufd.h
> +++ b/include/linux/iommufd.h
> @@ -236,6 +236,9 @@ int iommufd_object_depend(struct iommufd_object *obj_dependent,
> struct iommufd_object *obj_depended);
> void iommufd_object_undepend(struct iommufd_object *obj_dependent,
> struct iommufd_object *obj_depended);
> +int iommufd_ctx_alloc_mmap(struct iommufd_ctx *ictx, phys_addr_t base,
> + size_t size, unsigned long *immap_id);
> +void iommufd_ctx_free_mmap(struct iommufd_ctx *ictx, unsigned long immap_id);
> struct device *iommufd_viommu_find_dev(struct iommufd_viommu *viommu,
> unsigned long vdev_id);
> int iommufd_viommu_get_vdev_id(struct iommufd_viommu *viommu,
> @@ -262,11 +265,23 @@ static inline int iommufd_object_depend(struct iommufd_object *obj_dependent,
> return -EOPNOTSUPP;
> }
>
> +static inline int iommufd_ctx_alloc_mmap(struct iommufd_ctx *ictx,
> + phys_addr_t base, size_t size,
> + unsigned long *immap_id)
> +{
> + return -EOPNOTSUPP;
> +}
> +
> static inline void iommufd_object_undepend(struct iommufd_object *obj_dependent,
> struct iommufd_object *obj_depended)
> {
> }
>
> +static inline void iommufd_ctx_free_mmap(struct iommufd_ctx *ictx,
> + unsigned long immap_id)
> +{
> +}
> +
> static inline struct device *
> iommufd_viommu_find_dev(struct iommufd_viommu *viommu, unsigned long vdev_id)
> {
> diff --git a/drivers/iommu/iommufd/driver.c b/drivers/iommu/iommufd/driver.c
> index fb7f8fe40f95..c55336c580dc 100644
> --- a/drivers/iommu/iommufd/driver.c
> +++ b/drivers/iommu/iommufd/driver.c
> @@ -78,6 +78,45 @@ void iommufd_object_undepend(struct iommufd_object *obj_dependent,
> }
> EXPORT_SYMBOL_NS_GPL(iommufd_object_undepend, "IOMMUFD");
>
> +/* Driver should report the output @immap_id to user space for mmap() syscall */
> +int iommufd_ctx_alloc_mmap(struct iommufd_ctx *ictx, phys_addr_t base,
> + size_t size, unsigned long *immap_id)
> +{
> + struct iommufd_mmap *immap;
> + int rc;
> +
> + if (WARN_ON_ONCE(!immap_id))
> + return -EINVAL;
> + if (base & ~PAGE_MASK)
> + return -EINVAL;

Is it equal to PAGE_ALIGNED()?

> + if (!size || size & ~PAGE_MASK)
> + return -EINVAL;
> +
> + immap = kzalloc(sizeof(*immap), GFP_KERNEL);
> + if (!immap)
> + return -ENOMEM;
> + immap->pfn_start = base >> PAGE_SHIFT;
> + immap->pfn_end = immap->pfn_start + (size >> PAGE_SHIFT) - 1;
> +
> + rc = mtree_alloc_range(&ictx->mt_mmap, immap_id, immap, sizeof(immap),
> + 0, LONG_MAX >> PAGE_SHIFT, GFP_KERNEL);
> + if (rc < 0) {
> + kfree(immap);
> + return rc;
> + }
> +
> + /* mmap() syscall will right-shift the immap_id to vma->vm_pgoff */
> + *immap_id <<= PAGE_SHIFT;
> + return 0;
> +}
> +EXPORT_SYMBOL_NS_GPL(iommufd_ctx_alloc_mmap, "IOMMUFD");
> +
> +void iommufd_ctx_free_mmap(struct iommufd_ctx *ictx, unsigned long immap_id)
> +{
> + kfree(mtree_erase(&ictx->mt_mmap, immap_id >> PAGE_SHIFT));

MMIO lifecycle question: what happens if a region is removed from the
maple tree (and is therefore no longer mappable), but is still mapped
and in use by userspace?

> +}
> +EXPORT_SYMBOL_NS_GPL(iommufd_ctx_free_mmap, "IOMMUFD");
> +
> /* Caller should xa_lock(&viommu->vdevs) to protect the return value */
> struct device *iommufd_viommu_find_dev(struct iommufd_viommu *viommu,
> unsigned long vdev_id)

Thanks,
baolu