Subject: Re: [PATCH v4 04/13] dma: swiotlb: track pool encryption state and honor DMA_ATTR_CC_SHARED

From: Aneesh Kumar K.V <[email-masked]>
Date: Wed, 20 May 2026 09:27:27 +0530

Jason Gunthorpe <[email-masked]> writes:

> On Tue, May 19, 2026 at 09:35:30PM +0530, Aneesh Kumar K.V wrote:
>> Yes, that also resulted in simpler and cleaner code.
>> 
>> swiotlb_tbl_map_single
>> 	/*
>> 	 * If the physical address is encrypted but the device requires
>> 	 * decrypted DMA, use a decrypted io_tlb_mem and update the
>> 	 * attributes so the caller knows that a decrypted io_tlb_mem
>> 	 * was used.
>> 	 */
>> 	if (!(*attrs & DMA_ATTR_CC_SHARED) && force_dma_unencrypted(dev))
>> 		*attrs |= DMA_ATTR_CC_SHARED;
>> 
>> 	if (mem->unencrypted != !!(*attrs & DMA_ATTR_CC_SHARED))
>> 		return (phys_addr_t)DMA_MAPPING_ERROR;
>
> Yeah, exactly that is so much clearer now that the mem->unecrypted is
> tied directly.
>
> That logic is reversed though, the incoming ATTR_CC doesn't matter for
> swiotlb, that is just the source of the memcpy.
>
> /* swiotlb pool is incorrect for this device */
> if (mem->unencrypted != force_dma_unencrypted(dev))
>     return (phys_addr_t)DMA_MAPPING_ERROR;
>
> /* Force attrs to match the kind of memory in the pool */
> if (mem->unencrypted)
>      *attrs |= DMA_ATTR_CC_SHARED;
> else
>      *attrs &= ~DMA_ATTR_CC_SHARED;
>
>
> Attrs should be forced to whatever memory swiotlb selected.
>

But that will not handle a T=1 device that wants to use swiotlb to
bounce unencrypted memory. That is:

force_dma_unencrypted(dev) == 0  /* T=1 device */
attrs = DMA_ATTR_CC_SHARED;

In that case, it should use an unencrypted io_tlb_mem:
mem->unencrypted == 1

-aneesh