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

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Catalin Marinas <catalin.marinas@arm.com> writes:

> 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.
>

Considering for AMD/SME system an unencrypted dma addr is one without C
bit, will this be good?

	/*
	 * 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;
	}


>
> 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

-aneesh