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

[Jason Gunthorpe <jgg@ziepe.ca>]

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: