Re: [PATCH v7 16/22] dma-direct: make dma_direct_map_phys() honor DMA_ATTR_CC_SHARED

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Jul 10, 2026 at 10:52:49AM +0530, Aneesh Kumar K.V wrote:
> >> > 	/*
> >> > 	 * For host memory encryption and device requiring unencrypted DMA,
> >> > 	 * MMIO memory is treated as shared by default.
> >> > 	 */
> >> > 	if (attrs & DMA_ATTR_MMIO) {
> >> > 		if (cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT) || force_dma_unencrypted(dev))
> >> > 			attrs |= DMA_ATTR_CC_SHARED;
> >> > 	}
> >> 
> >> Yes, I think it does the trick, preserves the current semantics for AMD.
> >> I guess you could use a single 'if' for all checks (up to you).
> >
> > Please don't change it, MMIO P2P is broken on CC systems today and it
> > should stay broken. Passing DMA_ATTR_MMIO with DMA_ATTR_CC_SHARED is
> > an error that we need to correct in the drivers not make work in the
> > core code.
> >
> 
> But the above changes are intended to handle HOST_MEM_ENCRYPT. In v7, we
> had the following diff:

To follow how the rest of the decrypted/encrypted stuff works the MMIO
has to be flaged with CC_SHARED for HOST_MEM_ENCRYPT too, just like
the PTEs.

> @@ -88,37 +88,40 @@ static inline dma_addr_t dma_direct_map_phys(struct device *dev,
>  {
>  	dma_addr_t dma_addr;
>  
> +	/*
> +	 * For a device requiring unencrypted DMA, MMIO memory is treated
> +	 * as shared by default.
> +	 */
> +	if (force_dma_unencrypted(dev) && (attrs & DMA_ATTR_MMIO))
> +		attrs |= DMA_ATTR_CC_SHARED;

force_dma_unencrypted() says nothing about the properties of the
address passed in, this was nonsense :\

> As we discussed [1], that can come in a later patch. In the meantime, adding
> the HOST_MEM_ENCRYPT check preserves the previous behavior for SME.

It never worked. When we added ATTR_MMIO it started to have a chance
to work but prior to that it was always broken anyhow. I don't see
there is much merit in preserving the narrow window when we
inadvertantly had a half working ATTR_MMIO.

But if you really want to it should be
cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT) *only* and get rid of the wrong
force_dma_unencrypted().

But IMHO, I'd rather this series treat ATTR_MMIO as private MMIO and
ATTR_MMIO|CC_SHARED as shared MMIO and that's the right and correct
thing for the DMA API.

Jason