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

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

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.

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

-- 
Catalin