Claude review: dma-mapping: introduce DMA_ATTR_CC_DECRYPTED for pre-decrypted memory

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

The new attribute definition and trace event addition are straightforward.

> +	if (is_swiotlb_force_bounce(dev)) {
> +		if (!(attrs & DMA_ATTR_CC_DECRYPTED)) {
> +			if (attrs & DMA_ATTR_MMIO)
> +				return DMA_MAPPING_ERROR;
> +
> +			return swiotlb_map(dev, phys, size, dir, attrs);
> +		}
> +	} else if (attrs & DMA_ATTR_CC_DECRYPTED) {
> +		return DMA_MAPPING_ERROR;
> 	}

The logic here is correct: CC_DECRYPTED bypasses bounce buffering when force-bounce is active (the CoCo case), and returns an error when force-bounce is not active (CC_DECRYPTED makes no sense outside CoCo). The existing MMIO and swiotlb paths are unchanged when CC_DECRYPTED is not set.

> +	} else if (attrs & DMA_ATTR_CC_DECRYPTED) {
> +		dma_addr = phys_to_dma_unencrypted(dev, phys);
> +		if (unlikely(!dma_capable(dev, dma_addr, size, false)))
> +			goto err_overflow;

Using `phys_to_dma_unencrypted()` here is correct -- it produces a DMA address without the encryption bit, matching how swiotlb buffers are mapped. Passing `false` to `dma_capable` (no need to check against the encryption-adjusted mask) follows the same pattern as MMIO.

One thing worth discussing: `dma_direct_unmap_phys` is not updated to handle `DMA_ATTR_CC_DECRYPTED`. The unmap path calls `dma_to_phys(dev, addr)` on the unencrypted DMA address and then calls `swiotlb_tbl_unmap_single()`. This works because `dma_addr_canonical()` correctly strips encryption bits and `swiotlb_tbl_unmap_single` will find no matching pool and return. But it is slightly different from MMIO which gets an early return. Is there a reason not to add a similar early return for CC_DECRYPTED, or should the CPU sync still run for these pages (which are cacheable, unlike MMIO)?

Similarly, the `arch_sync_dma_for_device` call after the mapping succeeds does not exclude CC_DECRYPTED:

> 	if (!dev_is_dma_coherent(dev) &&
> 	    !(attrs & (DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_MMIO)))
> 		arch_sync_dma_for_device(phys, size, dir);

Since CC_DECRYPTED pages are normal cacheable memory, including them in the sync is the right behavior. This is fine.

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