Re: [RFC PATCH 2/2] dma-mapping: Use the correct phys_to_dma() for DMA_RESTRICTED_POOL

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 10/03/2026 13:08, Catalin Marinas wrote:
> On Thu, Mar 05, 2026 at 05:03:35PM +0000, Mostafa Saleh wrote:
>> As restricted dma pools are always decrypted, in swiotlb.c it uses
>> phys_to_dma_unencrypted() for address conversion.
>>
>> However, in DMA-direct, calls to phys_to_dma_direct() with
>> force_dma_unencrypted() returning false, will fallback to
>> phys_to_dma() which is inconsistent for memory allocated from
>> restricted dma pools.
>>
>> Signed-off-by: Mostafa Saleh <smostafa@google.com>
>> ---
>>   kernel/dma/direct.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
>> index 27d804f0473f..1a402bb956d9 100644
>> --- a/kernel/dma/direct.c
>> +++ b/kernel/dma/direct.c
>> @@ -26,7 +26,7 @@ u64 zone_dma_limit __ro_after_init = DMA_BIT_MASK(24);
>>   static inline dma_addr_t phys_to_dma_direct(struct device *dev,
>>   		phys_addr_t phys)
>>   {
>> -	if (force_dma_unencrypted(dev))
>> +	if (force_dma_unencrypted(dev) || is_swiotlb_for_alloc(dev))
>>   		return phys_to_dma_unencrypted(dev, phys);
>>   	return phys_to_dma(dev, phys);
>>   }
> 
> I couldn't fully get my head around the DMA API but I think all the
> pools and bounce buffers are decrypted and protected guests (or realms
> for Arm CCA) should always return true for force_dma_unencrypted(). If
> that's the case, the above change wouldn't be necessary. I can see that
> arm64 only does this for CCA and not pKVM guests.

That is correct. Why would the force_dma_unencrypted() return false for
a device ?  As far as I can see, all CC guests are treating all devices
as "untrusted" for now (and there is a series available that is adding
support for "trusted devices [0]).


> 
> Device assignment is another story that requires reworking those DMA
> pools to support encrypted buffers.
> 

[0] 
https://lkml.kernel.org/r/20260303000207.1836586-1-dan.j.williams@intel.com

Suzuki