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

Jiri Pirko
2026-04-20T09:29:29Z

Mon, Apr 20, 2026 at 08:34:06AM +0200, aneesh.kumar@kernel.org wrote:
>Jiri Pirko <jiri@resnulli.us> writes:
>
>> 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;
>> }
>>
>
>What is this check for? If we are requesting a DMA mapping with
>DMA_ATTR_CC_SHARED, shouldn’t it be allowed? If not, how would we reach

This is defensive. Only allows to map with DMA_ATTR_CC_SHARED set to
dev dev that does not support CC natively. This can be of course lifted,
if you have a case.

>the conditional below where we convert the physical address to a DMA
>address using phys_to_dma_unencrypted()?. Also, how is this supposed to
>interact with is_swiotlb_force_bounce()?”

You reach there when is_swiotlb_force_bounce(dev) is true and
DMA_ATTR_CC_SHARED is set. What am I missing?

>
>>
>> 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)) ||
>
>-aneesh