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

From: Jason Gunthorpe <[email-masked]>
Date: Wed, 20 May 2026 10:40:39 -0300

On Wed, May 20, 2026 at 09:27:27AM +0530, Aneesh Kumar K.V wrote:
> 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

No! The DMA_ATTR_CC_SHARED only states the nature of the source
memory, the DMA transfer will always happen under T=1

It is perfectly fine to memcpy from shared to private and do a T=1 DMA
from the private memory if we have to bounce.

Jason