Re: [PATCH v5 1/2] dma-mapping: introduce DMA_ATTR_CC_SHARED for shared memory

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Jason Gunthorpe <jgg@ziepe.ca> writes:

> On Thu, May 21, 2026 at 09:05:39PM +0530, Aneesh Kumar K.V wrote:
>> I am wondering whether this is better
>> 
>> static inline dma_addr_t dma_direct_map_phys(struct device *dev,
>> 		phys_addr_t phys, size_t size, enum dma_data_direction dir,
>> 		unsigned long attrs, bool flush)
>> {
>> 	dma_addr_t dma_addr;
>> 
>> 	/*
>> 	 * For a device requiring unencrypted DMA, MMIO memory is treated
>> 	 * as shared.
>> 	 */
>> 	if (force_dma_unencrypted(dev) && (attrs & DMA_ATTR_MMIO))
>> 		attrs |= DMA_ATTR_CC_SHARED;
>
> It is an option, I would be happier if we went and fixed the few
> callers to properly pass the shared. CC did this with the
> pgprot_decrypted() stuff, same reasoning:
>
> diff --git a/block/blk-mq-dma.c b/block/blk-mq-dma.c
> index bfdb9ed7074116..e77f6404caa3db 100644
> --- a/block/blk-mq-dma.c
> +++ b/block/blk-mq-dma.c
> @@ -90,7 +90,7 @@ static bool blk_dma_map_direct(struct request *req, struct device *dma_dev,
>  	unsigned int attrs = 0;
>  
>  	if (iter->p2pdma.map == PCI_P2PDMA_MAP_THRU_HOST_BRIDGE)
> -		attrs |= DMA_ATTR_MMIO;
> +		attrs |= iter->p2pdma.mem->dma_mapping_flags;
>  
>  	iter->addr = dma_map_phys(dma_dev, vec->paddr, vec->len,
>  			rq_dma_dir(req), attrs);
> @@ -115,7 +115,7 @@ static bool blk_rq_dma_map_iova(struct request *req, struct device *dma_dev,
>  	iter->len = dma_iova_size(state);
>  
>  	if (iter->p2pdma.map == PCI_P2PDMA_MAP_THRU_HOST_BRIDGE)
> -		attrs |= DMA_ATTR_MMIO;
> +		attrs |= iter->p2pdma.mem->dma_mapping_flags;
>  
>  	do {
>  		error = dma_iova_link(dma_dev, state, vec->paddr, mapped,
> diff --git a/drivers/dma-buf/dma-buf-mapping.c b/drivers/dma-buf/dma-buf-mapping.c
> index 794acff2546a34..96022fadc48245 100644
> --- a/drivers/dma-buf/dma-buf-mapping.c
> +++ b/drivers/dma-buf/dma-buf-mapping.c
> @@ -147,7 +147,7 @@ struct sg_table *dma_buf_phys_vec_to_sgt(struct dma_buf_attachment *attach,
>  			ret = dma_iova_link(attach->dev, dma->state,
>  					    phys_vec[i].paddr, 0,
>  					    phys_vec[i].len, dir,
> -					    DMA_ATTR_MMIO);
> +					    provider->dma_mapping_flags);
>  			if (ret)
>  				goto err_unmap_dma;
>  
> @@ -155,7 +155,7 @@ struct sg_table *dma_buf_phys_vec_to_sgt(struct dma_buf_attachment *attach,
>  		} else {
>  			addr = dma_map_phys(attach->dev, phys_vec[i].paddr,
>  					    phys_vec[i].len, dir,
> -					    DMA_ATTR_MMIO);
> +					    provider->dma_mapping_flags);
>  			ret = dma_mapping_error(attach->dev, addr);
>  			if (ret)
>  				goto err_unmap_dma;
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 7c898542af8d5e..e4229b4d35c767 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -282,6 +282,8 @@ int pcim_p2pdma_init(struct pci_dev *pdev)
>  			continue;
>  
>  		p2p->mem[i].owner = &pdev->dev;
> +		p2p->mem[i].dma_mapping_flags =
> +			DMA_ATTR_MMIO | DMA_ATTR_CC_SHARED;
>  		p2p->mem[i].bus_offset =
>  			pci_bus_address(pdev, i) - pci_resource_start(pdev, i);
>  	}
> diff --git a/include/linux/pci-p2pdma.h b/include/linux/pci-p2pdma.h
> index 873de20a224759..402dc5e5d62b0a 100644
> --- a/include/linux/pci-p2pdma.h
> +++ b/include/linux/pci-p2pdma.h
> @@ -21,10 +21,12 @@ struct scatterlist;
>   *
>   * A p2pdma provider is a range of MMIO address space available to the CPU.
>   * @owner: Device to which this provider belongs.
> + * @dma_mapping_flags: DMA attributes to use for host bridge mappings.
>   * @bus_offset: Bus offset for p2p communication.
>   */
>  struct p2pdma_provider {
>  	struct device *owner;
> +	unsigned long dma_mapping_flags;
>  	u64 bus_offset;
>  };
>  
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 5955f2f0c83db1..c3f445acddf873 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -811,7 +811,7 @@ dma_addr_t hmm_dma_map_pfn(struct device *dev, struct hmm_dma_map *map,
>  	case PCI_P2PDMA_MAP_NONE:
>  		break;
>  	case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> -		attrs |= DMA_ATTR_MMIO;
> +		attrs |= p2pdma_state->mem->dma_mapping_flags;
>  		pfns[idx] |= HMM_PFN_P2PDMA;
>  		break;
>  	case PCI_P2PDMA_MAP_BUS_ADDR:

Can I convert this as an independent patch with your SOB?

-aneesh