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

Jiri Pirko
2026-03-25T19:23:56Z

From: Jiri Pirko <jiri@nvidia.com>

Current CC designs don't place a vIOMMU in front of untrusted devices.
Instead, the DMA API forces all untrusted device DMA through swiotlb
bounce buffers (is_swiotlb_force_bounce()) which copies data into
shared memory on behalf of the device.

When a caller has already arranged for the memory to be shared
via set_memory_decrypted(), the DMA API needs to know so it can map
directly using the unencrypted physical address rather than bounce
buffering. Following the pattern of DMA_ATTR_MMIO, add
DMA_ATTR_CC_SHARED for this purpose. Like the MMIO case, only the
caller knows what kind of memory it has and must inform the DMA API
for it to work correctly.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
---
v4->v5:
- rebased on top od dma-mapping-for-next
- s/decrypted/shared/
v3->v4:
- added some sanity checks to dma_map_phys and dma_unmap_phys
- enhanced documentation of DMA_ATTR_CC_DECRYPTED attr
v1->v2:
- rebased on top of recent dma-mapping-fixes
---
include/linux/dma-mapping.h | 10 ++++++++++
include/trace/events/dma.h | 3 ++-
kernel/dma/direct.h | 14 +++++++++++---
kernel/dma/mapping.c | 13 +++++++++++--
4 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 677c51ab7510..db8ab24a54f4 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -92,6 +92,16 @@
* flushing.
*/
#define DMA_ATTR_REQUIRE_COHERENT (1UL << 12)
+/*
+ * DMA_ATTR_CC_SHARED: Indicates the DMA mapping is shared (decrypted) for
+ * confidential computing guests. For normal system memory the caller must have
+ * called set_memory_decrypted(), and pgprot_decrypted must be used when
+ * creating CPU PTEs for the mapping. The same shared semantic may be passed
+ * to the vIOMMU when it sets up the IOPTE. For MMIO use together with
+ * DMA_ATTR_MMIO to indicate shared MMIO. Unless DMA_ATTR_MMIO is provided
+ * a struct page is required.
+ */
+#define DMA_ATTR_CC_SHARED (1UL << 13)

/*
* A dma_addr_t can hold any valid DMA or bus address for the platform. It can
diff --git a/include/trace/events/dma.h b/include/trace/events/dma.h
index 63597b004424..31c9ddf72c9d 100644
--- a/include/trace/events/dma.h
+++ b/include/trace/events/dma.h
@@ -34,7 +34,8 @@ TRACE_DEFINE_ENUM(DMA_NONE);
{ DMA_ATTR_PRIVILEGED, "PRIVILEGED" }, \
{ DMA_ATTR_MMIO, "MMIO" }, \
{ DMA_ATTR_DEBUGGING_IGNORE_CACHELINES, "CACHELINES_OVERLAP" }, \
- { DMA_ATTR_REQUIRE_COHERENT, "REQUIRE_COHERENT" })
+ { DMA_ATTR_REQUIRE_COHERENT, "REQUIRE_COHERENT" }, \
+ { DMA_ATTR_CC_SHARED, "CC_SHARED" })

DECLARE_EVENT_CLASS(dma_map,
TP_PROTO(struct device *dev, phys_addr_t phys_addr, dma_addr_t dma_addr,
diff --git a/kernel/dma/direct.h b/kernel/dma/direct.h
index b86ff65496fc..7140c208c123 100644
--- a/kernel/dma/direct.h
+++ b/kernel/dma/direct.h
@@ -89,16 +89,24 @@ static inline dma_addr_t dma_direct_map_phys(struct device *dev,
dma_addr_t dma_addr;

if (is_swiotlb_force_bounce(dev)) {
- if (attrs & (DMA_ATTR_MMIO | DMA_ATTR_REQUIRE_COHERENT))
- return DMA_MAPPING_ERROR;
+ if (!(attrs & DMA_ATTR_CC_SHARED)) {
+ if (attrs & (DMA_ATTR_MMIO | DMA_ATTR_REQUIRE_COHERENT))
+ return DMA_MAPPING_ERROR;

- return swiotlb_map(dev, phys, size, dir, attrs);
+ return swiotlb_map(dev, phys, size, dir, attrs);
+ }
+ } else if (attrs & DMA_ATTR_CC_SHARED) {
+ return DMA_MAPPING_ERROR;
}

if (attrs & DMA_ATTR_MMIO) {
dma_addr = phys;
if (unlikely(!dma_capable(dev, dma_addr, size, false)))
goto err_overflow;
+ } else if (attrs & DMA_ATTR_CC_SHARED) {
+ dma_addr = phys_to_dma_unencrypted(dev, phys);
+ if (unlikely(!dma_capable(dev, dma_addr, size, false)))
+ goto err_overflow;
} else {
dma_addr = phys_to_dma(dev, phys);
if (unlikely(!dma_capable(dev, dma_addr, size, true)) ||
diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index df3eccc7d4ca..23ed8eb9233e 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -157,6 +157,7 @@ dma_addr_t dma_map_phys(struct device *dev, phys_addr_t phys, size_t size,
{
const struct dma_map_ops *ops = get_dma_ops(dev);
bool is_mmio = attrs & DMA_ATTR_MMIO;
+ bool is_cc_shared = attrs & DMA_ATTR_CC_SHARED;
dma_addr_t addr = DMA_MAPPING_ERROR;

BUG_ON(!valid_dma_direction(dir));
@@ -168,8 +169,11 @@ dma_addr_t dma_map_phys(struct device *dev, phys_addr_t phys, size_t size,
return DMA_MAPPING_ERROR;

if (dma_map_direct(dev, ops) ||
- (!is_mmio && arch_dma_map_phys_direct(dev, phys + size)))
+ (!is_mmio && !is_cc_shared &&
+ arch_dma_map_phys_direct(dev, phys + size)))
addr = dma_direct_map_phys(dev, phys, size, dir, attrs, true);
+ else if (is_cc_shared)
+ return DMA_MAPPING_ERROR;
else if (use_dma_iommu(dev))
addr = iommu_dma_map_phys(dev, phys, size, dir, attrs);
else if (ops->map_phys)
@@ -206,11 +210,16 @@ void dma_unmap_phys(struct device *dev, dma_addr_t addr, size_t size,
{
const struct dma_map_ops *ops = get_dma_ops(dev);
bool is_mmio = attrs & DMA_ATTR_MMIO;
+ bool is_cc_shared = attrs & DMA_ATTR_CC_SHARED;

BUG_ON(!valid_dma_direction(dir));
+
if (dma_map_direct(dev, ops) ||
- (!is_mmio && arch_dma_unmap_phys_direct(dev, addr + size)))
+ (!is_mmio && !is_cc_shared &&
+ arch_dma_unmap_phys_direct(dev, addr + size)))
dma_direct_unmap_phys(dev, addr, size, dir, attrs, true);
+ else if (is_cc_shared)
+ return;
else if (use_dma_iommu(dev))
iommu_dma_unmap_phys(dev, addr, size, dir, attrs);
else if (ops->unmap_phys)
--
2.51.1