Re: [PATCH RFC 21/26] dma-buf: Add the Physical Address List DMA mapping type

["Christian König" <christian.koenig@amd.com>]

On 2/18/26 01:11, Jason Gunthorpe wrote:
> This type is required by iommufd and kvm as dmabuf importers.
> 
> Due to sensitivity about abusing physical addresses, restrict importers by
> using EXPORT_SYMBOL_FOR_MODULES(). Only iommufd can implement an importer,
> the kernel module loader will enforce this.
> 
> Allow anything to implement an exporter as there are use cases in
> DPDK/SPDK to connect GPU memory into VFIO/iommufd and it is hard to abuse
> the API as an exporter.
> 
> The physical address list exporter returns a physical address list in a
> simple kvalloc'd array of struct phys_vec.

As far as I can see that is still a pretty big NO-GO.

We have seen so many problems with direct physical address access by the importer that I clear don't want to repeat that performance.

My main question is why does IOMMUFD need the physical address in the first place?

If that is really strictly necessary then I strongly suggest to not touch drivers/dma-buf in any way, but only do this is private interface between iommufd and KVM.

Regards,
Christian.

> 
> For now all entries are assumed to be MMIO and iommufd will map into
> the IOMMU using the IOMMU_MMIO flag.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/dma-buf/dma-buf-mapping.c | 63 +++++++++++++++++++++++++++++++
>  include/linux/dma-buf-mapping.h   | 42 +++++++++++++++++++++
>  2 files changed, 105 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c
> index baa96b37e2c6bd..d9d6b9b5bf05c6 100644
> --- a/drivers/dma-buf/dma-buf-mapping.c
> +++ b/drivers/dma-buf/dma-buf-mapping.c
> @@ -349,3 +349,66 @@ struct dma_buf_mapping_type dma_buf_mapping_sgt_type = {
>  	.debugfs_dump = dma_buf_sgt_debugfs_dump,
>  };
>  EXPORT_SYMBOL_NS_GPL(dma_buf_mapping_sgt_type, "DMA_BUF");
> +
> +static const struct dma_buf_mapping_pal_exp_ops *
> +to_pal_exp_ops(struct dma_buf_attachment *attach)
> +{
> +	return container_of(attach->map_type.exp_ops,
> +			    struct dma_buf_mapping_pal_exp_ops, ops);
> +}
> +
> +/**
> + * dma_buf_pal_map_phys - Obtain the physical address list for a PAL attachment
> + * @attach: The DMA-buf attachment
> + *
> + * Calls the exporter's map_phys() callback to retrieve the physical address
> + * list for the buffer. The caller must hold the dma-buf's reservation lock.
> + *
> + * This symbol is restricted to iommufd to prevent misuse.
> + *
> + * Returns the physical address list on success, or an ERR_PTR on failure.
> + * The returned list must be freed with dma_buf_pal_unmap_phys().
> + */
> +struct dma_buf_phys_list *
> +dma_buf_pal_map_phys(struct dma_buf_attachment *attach)
> +{
> +	dma_resv_assert_held(attach->dmabuf->resv);
> +	return to_pal_exp_ops(attach)->map_phys(attach);
> +}
> +/*
> + * Restricted, iommufd is the only importer allowed to prevent misuse of this
> + * API.
> + */
> +EXPORT_SYMBOL_FOR_MODULES(dma_buf_pal_map_phys, "iommufd");
> +
> +/**
> + * dma_buf_pal_unmap_phys - Unmap a physical address list
> + * @attach: The DMA-buf attachment
> + * @phys: The physical address list returned by dma_buf_pal_map_phys()
> + *
> + * Returns the mapping back to the exporter. After this point the importer may
> + * not touch any of the addresses in any way.
> + */
> +void dma_buf_pal_unmap_phys(struct dma_buf_attachment *attach,
> +			    struct dma_buf_phys_list *phys)
> +{
> +	to_pal_exp_ops(attach)->unmap_phys(attach, phys);
> +}
> +EXPORT_SYMBOL_NS_GPL(dma_buf_pal_unmap_phys, "DMA_BUF");
> +
> +static inline void
> +dma_buf_pal_finish_match(struct dma_buf_match_args *args,
> +			 const struct dma_buf_mapping_match *exp,
> +			 const struct dma_buf_mapping_match *imp)
> +{
> +	args->attach->map_type = (struct dma_buf_mapping_match){
> +		.type = &dma_buf_mapping_pal_type,
> +		.exp_ops = exp->exp_ops,
> +	};
> +}
> +
> +struct dma_buf_mapping_type dma_buf_mapping_pal_type = {
> +	.name = "Physical Address List",
> +	.finish_match = dma_buf_pal_finish_match,
> +};
> +EXPORT_SYMBOL_NS_GPL(dma_buf_mapping_pal_type, "DMA_BUF");
> diff --git a/include/linux/dma-buf-mapping.h b/include/linux/dma-buf-mapping.h
> index ac859b8913edcd..10831ce2e72851 100644
> --- a/include/linux/dma-buf-mapping.h
> +++ b/include/linux/dma-buf-mapping.h
> @@ -269,4 +269,46 @@ DMA_BUF_EMAPPING_SGT_P2P(const struct dma_buf_mapping_sgt_exp_ops *exp_ops,
>  			.exporter_requires_p2p = DMA_SGT_NO_P2P,         \
>  		} })
>  
> +/*
> + * Physical Address List mapping type
> + *
> + * Use of the Physical Address List type is restricted to prevent abuse of the
> + * physical addresses API. Please check with the DMA BUF maintainers before
> + * trying to use it.
> + */
> +struct dma_buf_phys_list {
> +	size_t length;
> +	struct dma_buf_phys_vec phys[] __counted_by(length);
> +};
> +
> +extern struct dma_buf_mapping_type dma_buf_mapping_pal_type;
> +
> +struct dma_buf_mapping_pal_exp_ops {
> +	struct dma_buf_mapping_exp_ops ops;
> +	struct dma_buf_phys_list *(*map_phys)(struct dma_buf_attachment *attach);
> +	void (*unmap_phys)(struct dma_buf_attachment *attach,
> +			   struct dma_buf_phys_list *phys);
> +};
> +
> +struct dma_buf_phys_list *
> +dma_buf_pal_map_phys(struct dma_buf_attachment *attach);
> +void dma_buf_pal_unmap_phys(struct dma_buf_attachment *attach,
> +			    struct dma_buf_phys_list *phys);
> +
> +static inline struct dma_buf_mapping_match DMA_BUF_IMAPPING_PAL(void)
> +{
> +	return (struct dma_buf_mapping_match){
> +		.type = &dma_buf_mapping_pal_type,
> +	};
> +}
> +
> +static inline struct dma_buf_mapping_match
> +DMA_BUF_EMAPPING_PAL(const struct dma_buf_mapping_pal_exp_ops *exp_ops)
> +{
> +	return (struct dma_buf_mapping_match){
> +		.type = &dma_buf_mapping_pal_type,
> +		.exp_ops = &exp_ops->ops,
> +	};
> +}
> +
>  #endif