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

Aneesh Kumar K.V
2026-04-21T09:42:17Z

Jiri Pirko <jiri@resnulli.us> writes:

> 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?
>

So a swiotlb_force_bounce will not use swiotlb bouncing if
DMA_ATTR_CC_SHARED is set ?

>
>
>>
>>>
>>> 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