[PATCH v7 16/22] dma-direct: make dma_direct_map_phys() honor DMA_ATTR_CC_SHARED

[Aneesh Kumar K.V]

Jason Gunthorpe <jgg at ziepe.ca> writes:

> On Thu, Jul 09, 2026 at 12:13:19PM +0100, Catalin Marinas wrote:
>> > > For AMD/SME, on host with memory encryption we now end up setting the C
>> > > bit for DMA_ATTR_MMIO. This is fine for RAM but not sure whether
>> > > some other MMIO bus understands this attribute. Maybe we should stick to
>> > > something like __phys_to_dma() for the !CC_SHARED && MMIO path. Or,
>> > > since this is not universally defined, just use the old dma_addr = phys
>> > > if MMIO and ignore any unlikely DMA offsets.
>> > >
>> > 
>> > Considering for AMD/SME system an unencrypted dma addr is one without C
>> > bit, will this be good?
>> > 
>> > 	/*
>> > 	 * For host memory encryption and device requiring unencrypted DMA,
>> > 	 * MMIO memory is treated as shared by default.
>> > 	 */
>> > 	if (attrs & DMA_ATTR_MMIO) {
>> > 		if (cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT) || force_dma_unencrypted(dev))
>> > 			attrs |= DMA_ATTR_CC_SHARED;
>> > 	}
>> 
>> Yes, I think it does the trick, preserves the current semantics for AMD.
>> I guess you could use a single 'if' for all checks (up to you).
>
> Please don't change it, MMIO P2P is broken on CC systems today and it
> should stay broken. Passing DMA_ATTR_MMIO with DMA_ATTR_CC_SHARED is
> an error that we need to correct in the drivers not make work in the
> core code.
>

But the above changes are intended to handle HOST_MEM_ENCRYPT. In v7, we
had the following diff:

@@ -88,37 +88,40 @@ static inline dma_addr_t dma_direct_map_phys(struct device *dev,
 {
 	dma_addr_t dma_addr;
 
+	/*
+	 * For a device requiring unencrypted DMA, MMIO memory is treated
+	 * as shared by default.
+	 */
+	if (force_dma_unencrypted(dev) && (attrs & DMA_ATTR_MMIO))
+		attrs |= DMA_ATTR_CC_SHARED;
+

This is now getting updated to

@@ -624,37 +626,44 @@ dma_addr_t dma_direct_map_phys(struct device *dev, phys_addr_t phys,
 {
 	dma_addr_t dma_addr;
 
+	if (attrs & DMA_ATTR_MMIO) {
+		/*
+		 * For host memory encryption and device requiring
+		 * unencrypted DMA, MMIO memory is treated as shared by
+		 * default.
+		 */
+		if (cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT) ||
+		    force_dma_unencrypted(dev))
+			attrs |= DMA_ATTR_CC_SHARED;
+	}
+

I agree that we need to move that DMA_ATTR_CC_SHARED setting to 

modified   drivers/pci/p2pdma.c
@@ -285,6 +285,11 @@ int pcim_p2pdma_init(struct pci_dev *pdev)
 			continue;
 
 		p2p->mem[i].owner = &pdev->dev;
+
+		p2p->mem[i].dma_mapping_flags = DMA_ATTR_MMIO;
+		if (force_dma_unencrypted(dev))
+			p2p->mem[i].dma_mapping_flags |= DMA_ATTR_CC_SHARED;
+

As we discussed [1], that can come in a later patch. In the meantime, adding
the HOST_MEM_ENCRYPT check preserves the previous behavior for SME.

[1] https://lore.kernel.org/all/20260522132240.GD7702@ziepe.ca/

-aneesh