Re: [PATCH v2 6/8] dma-direct: make dma_direct_map_phys() honor DMA_ATTR_CC_SHARED

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

Aneesh Kumar K.V <aneesh.kumar@kernel.org> writes:

> Jason Gunthorpe <jgg@ziepe.ca> writes:
>
>> On Mon, Apr 20, 2026 at 11:44:13AM +0530, Aneesh Kumar K.V (Arm) wrote:
>>> Teach dma_direct_map_phys() to select the DMA address encoding based on
>>> DMA_ATTR_CC_SHARED.
>>> 
>>> Use phys_to_dma_unencrypted() for decrypted mappings and
>>> phys_to_dma_encrypted() otherwise. If a device requires unencrypted DMA
>>> but the source physical address is still encrypted, force the mapping
>>> through swiotlb so the DMA address and backing memory attributes remain
>>> consistent.
>>> 
>>> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
>>> ---
>>>  kernel/dma/direct.h | 25 ++++++++++++++-----------
>>>  1 file changed, 14 insertions(+), 11 deletions(-)
>>> 
>>> diff --git a/kernel/dma/direct.h b/kernel/dma/direct.h
>>> index 7140c208c123..928671ef01e9 100644
>>> --- a/kernel/dma/direct.h
>>> +++ b/kernel/dma/direct.h
>>> @@ -86,9 +86,14 @@ static inline dma_addr_t dma_direct_map_phys(struct device *dev,
>>>  		phys_addr_t phys, size_t size, enum dma_data_direction dir,
>>>  		unsigned long attrs, bool flush)
>>>  {
>>> +	bool force_swiotlb_map = false;
>>>  	dma_addr_t dma_addr;
>>>  
>>> -	if (is_swiotlb_force_bounce(dev)) {
>>> +	/* if phys addr attribute is encrypted but the device is forcing an encrypted dma addr */
>>> +	if (!(attrs & DMA_ATTR_CC_SHARED) && force_dma_unencrypted(dev))
>>> +		force_swiotlb_map = true;
>>
>> continuing my prior email.. This is really in the wrong spot, it
>> should be in dma_capable()
>>
>>> +	if (is_swiotlb_force_bounce(dev) || force_swiotlb_map) {
>>>  		if (!(attrs & DMA_ATTR_CC_SHARED)) {
>>>  			if (attrs & (DMA_ATTR_MMIO | DMA_ATTR_REQUIRE_COHERENT))
>>>  				return DMA_MAPPING_ERROR;
>>> @@ -105,18 +110,16 @@ static inline dma_addr_t dma_direct_map_phys(struct device *dev,
>>>  			goto err_overflow;
>>>  	} else if (attrs & DMA_ATTR_CC_SHARED) {
>>>  		dma_addr = phys_to_dma_unencrypted(dev, phys);
>>> -		if (unlikely(!dma_capable(dev, dma_addr, size, false)))
>>> -			goto err_overflow;
>>>  	} else {
>>> -		dma_addr = phys_to_dma(dev, phys);
>>> -		if (unlikely(!dma_capable(dev, dma_addr, size, true)) ||
>>> -		    dma_kmalloc_needs_bounce(dev, size, dir)) {
>>
>> here.
>>
>> swiotlb because the device can't reach a high address and swiotlb
>> because the device doesn't have T=1 are really the same thing and
>> should have the same code flow.
>>
>> Add attrs to dma_capable() and check force_dma_unencrypted(dev)
>> inside.
>>
>
> will update in the next revision
>
>>
>>> -			if (is_swiotlb_active(dev) &&
>>> -			    !(attrs & DMA_ATTR_REQUIRE_COHERENT))
>>> -				return swiotlb_map(dev, phys, size, dir, attrs);
>>> +		dma_addr = phys_to_dma_encrypted(dev, phys);
>>> +	}
>>>  
>>> -			goto err_overflow;
>>> -		}
>>> +	if (unlikely(!dma_capable(dev, dma_addr, size, true)) ||
>>> +	    dma_kmalloc_needs_bounce(dev, size, dir)) {
>>> +		if (is_swiotlb_active(dev) &&
>>> +		    !(attrs & DMA_ATTR_REQUIRE_COHERENT))
>>> +			return swiotlb_map(dev, phys, size, dir, attrs);
>>> +		goto err_overflow;
>>>  	}
>>
>> Then this movement shouldn't be needed?
>
> I am still not clear about the use of DMA_ATTR_CC_SHARED here. If the
> resulting DMA address is not dma_capable, I was expecting that we should
> fall back to swiotlb_map(). That was the intention behind this change.
> However, the other email thread suggests that DMA_ATTR_CC_SHARED is
> always used with swiotlb_force_bounce(). I think we should address that.
> If we do, the goal here would be to check dma_capable for both shared
> and private addresses.
>
> For private/protected addresses, swiotlb_map() will currently fail with
> DMA_MAPPING_ERROR because the default io_tlb_mem (dev->dma_io_tlb_mem)
> is decrypted by default


Also note that if a DMA_ATTR_CC_SHARED DMA address is not dma_capable,
then swiotlb_map() will most likely fail as well. We do check the
resulting SWIOTLB-mapped address again in swiotlb_map(). That is, if a
DMA_ATTR_CC_SHARED physical address fails because the resulting DMA
address exceeds the DMA mask (due to phys_to_dma_unencrypted() adding
the top bit), then the SWIOTLB-mapped DMA address will likely fail the
capability check for the same reason, as it would also include the top
bit.

So, if we want, we could avoid moving that if condition. However, IMHO,
we could make it more generic to indicate that if any address is not
DMA-capable, and if a SWIOTLB is active, we should attempt swiotlb_map()

-aneesh