Re: [PATCH v6 00/20] dma-mapping: Use DMA_ATTR_CC_SHARED through direct, pool and swiotlb paths

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, Jun 09, 2026 at 02:43:08PM +0100, Catalin Marinas wrote:
> On Thu, Jun 04, 2026 at 02:09:39PM +0530, Aneesh Kumar K.V (Arm) wrote:
> > This series propagates DMA_ATTR_CC_SHARED through the dma-direct,
> > dma-pool, and swiotlb paths so that encrypted and decrypted DMA buffers
> > are handled consistently.
> > 
> > Today, the direct DMA path mostly relies on force_dma_unencrypted() for
> > shared/decrypted buffer handling. This series consolidates the
> > force_dma_unencrypted() checks in the top-level functions and ensures
> > that the remaining DMA interfaces use DMA attributes to make the correct
> > decisions.
> 
> Please check Sashiko's reports, it has some good points:
> 
> https://sashiko.dev/#/patchset/20260604083959.1265923-1-aneesh.kumar@kernel.org
> 
> I think the main one is the swiotlb_tbl_map_single() changes which break
> AMD SME host support. There cc_platform_has(CC_ATTR_MEM_ENCRYPT) is true
> but force_dma_unencrypted() is false. Normally you'd not end up on this
> path but you can have swiotlb=force.

IMHO that's an AMD issue, not with the design of this series..

The series is right, a device that is !force_dma_decrypted() must be
considerd to be a trusted device and we must never place any DMA
mappings for a trusted device into shared memory.

That AMD has done somethine insane:

bool force_dma_unencrypted(struct device *dev)
{
	/*
	 * For SEV, all DMA must be to unencrypted addresses.
	 */
	if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT))
		return true;

	/*
	 * For SME, all DMA must be to unencrypted addresses if the
	 * device does not support DMA to addresses that include the
	 * encryption mask.
	 */
	if (cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT)) {
		u64 dma_enc_mask = DMA_BIT_MASK(__ffs64(sme_me_mask));
		u64 dma_dev_mask = min_not_zero(dev->coherent_dma_mask,
						dev->bus_dma_limit);

		if (dma_dev_mask <= dma_enc_mask)
			return true;
	}

Is an AMD issue. We already have an address mask limit system built
into the DMA API, arch code should not be co-opting the CC mechanism
to create a special pool for address limited devices.

The correct thing is to ensure the DMA API is checking any address
limits on the actual true dma_addr_t, not on an intermediate like a
phys_addr before it is adjusted with any C bit. Then it is a normal
low address swiotlb bounce like any other.

I think we can ignore this Sashiko remark, in real systems the use of
swiotlb for 64 bit devices is very rare. Though it would be good to
remove this code from AMD...

Jason