Catalin Marinas [off-list ref] writes:

On Wed, Jul 01, 2026 at 11:19:20AM +0530, Aneesh Kumar K.V (Arm) wrote:

quoted

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 97987f850a33..acf67c7064db 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c

@@ -338,10 +338,8 @@ void __init arch_mm_preinit(void)
 	unsigned int flags = SWIOTLB_VERBOSE;
 	bool swiotlb = max_pfn > PFN_DOWN(arm64_dma_phys_limit);
 
-	if (is_realm_world()) {
+	if (is_realm_world())
 		swiotlb = true;
-		flags |= SWIOTLB_FORCE;
-	}

For this part:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

quoted

diff --git a/kernel/dma/direct.h b/kernel/dma/direct.h
index e05dc7649366..f3fc28f352ba 100644
--- a/kernel/dma/direct.h
+++ b/kernel/dma/direct.h

@@ -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;
+
 	if (is_swiotlb_force_bounce(dev)) {
-		if (!(attrs & DMA_ATTR_CC_SHARED)) {
-			if (attrs & (DMA_ATTR_MMIO | DMA_ATTR_REQUIRE_COHERENT))
-				return DMA_MAPPING_ERROR;
+		if (attrs & (DMA_ATTR_MMIO | DMA_ATTR_REQUIRE_COHERENT))
+			return DMA_MAPPING_ERROR;
 
-			return swiotlb_map(dev, phys, size, dir, attrs);
-		}
-	} else if (attrs & DMA_ATTR_CC_SHARED) {
-		return DMA_MAPPING_ERROR;
+		return swiotlb_map(dev, phys, size, dir, attrs);
 	}
 
-	if (attrs & DMA_ATTR_MMIO) {
-		dma_addr = phys;
-		if (unlikely(!dma_capable(dev, dma_addr, size, false, attrs)))
-			goto err_overflow;
-	} else if (attrs & DMA_ATTR_CC_SHARED) {
+	if (attrs & DMA_ATTR_CC_SHARED)
 		dma_addr = phys_to_dma_unencrypted(dev, phys);
+	else
+		dma_addr = phys_to_dma_encrypted(dev, phys);

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;
	}

In the other case, for an arm CCA guest, if the MMIO is shared we end up
setting the shared attribute but that's fine, it's only an IPA address.

-- 
Catalin

-aneesh