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

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Jul 01, 2026 at 11:19:20AM +0530, Aneesh Kumar K.V (Arm) wrote:
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 97987f850a33..acf67c7064db 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -338,10 +338,8 @@ void __init arch_mm_preinit(void)
>  	unsigned int flags = SWIOTLB_VERBOSE;
>  	bool swiotlb = max_pfn > PFN_DOWN(arm64_dma_phys_limit);
>  
> -	if (is_realm_world()) {
> +	if (is_realm_world())
>  		swiotlb = true;
> -		flags |= SWIOTLB_FORCE;
> -	}

For this part:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

> diff --git a/kernel/dma/direct.h b/kernel/dma/direct.h
> index e05dc7649366..f3fc28f352ba 100644
> --- a/kernel/dma/direct.h
> +++ b/kernel/dma/direct.h
> @@ -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;
> +
>  	if (is_swiotlb_force_bounce(dev)) {
> -		if (!(attrs & DMA_ATTR_CC_SHARED)) {
> -			if (attrs & (DMA_ATTR_MMIO | DMA_ATTR_REQUIRE_COHERENT))
> -				return DMA_MAPPING_ERROR;
> +		if (attrs & (DMA_ATTR_MMIO | DMA_ATTR_REQUIRE_COHERENT))
> +			return DMA_MAPPING_ERROR;
>  
> -			return swiotlb_map(dev, phys, size, dir, attrs);
> -		}
> -	} else if (attrs & DMA_ATTR_CC_SHARED) {
> -		return DMA_MAPPING_ERROR;
> +		return swiotlb_map(dev, phys, size, dir, attrs);
>  	}
>  
> -	if (attrs & DMA_ATTR_MMIO) {
> -		dma_addr = phys;
> -		if (unlikely(!dma_capable(dev, dma_addr, size, false, attrs)))
> -			goto err_overflow;
> -	} else if (attrs & DMA_ATTR_CC_SHARED) {
> +	if (attrs & DMA_ATTR_CC_SHARED)
>  		dma_addr = phys_to_dma_unencrypted(dev, phys);
> +	else
> +		dma_addr = phys_to_dma_encrypted(dev, phys);

For AMD/SME, on host with memory encryption we now end up setting the C
bit for DMA_ATTR_MMIO. This is fine for RAM but not sure whether
some other MMIO bus understands this attribute. Maybe we should stick to
something like __phys_to_dma() for the !CC_SHARED && MMIO path. Or,
since this is not universally defined, just use the old dma_addr = phys
if MMIO and ignore any unlikely DMA offsets.

In the other case, for an arm CCA guest, if the MMIO is shared we end up
setting the shared attribute but that's fine, it's only an IPA address.

-- 
Catalin