Re: [PATCH] x86/mm: Don't force unencrypted DMA for IOMMU-backed devices

[Alexey Kardashevskiy <aik@amd.com>]

On 8/9/26 21:32, Aneesh Kumar K.V (Arm) wrote:
> Commit 8277a12d0d60 ("dma-pool: track decrypted atomic pools and select
> them via attrs") exposed an issue with force_dma_unencrypted() on
> systems using host memory encryption.
> 
> force_dma_unencrypted() checks whether the device DMA mask can address
> the encryption bit and, if not, requires DMA allocations to use
> unencrypted memory. However, this check is not applicable when the
> device is using the IOMMU. In that case, the device DMA mask constrains
> the IOVA seen by the device, not the backing physical address, so it
> does not need to cover the C-bit.
> 
> This currently causes dma_alloc_attrs() to set
> __DMA_ATTR_ALLOC_CC_SHARED for such devices. iommu_dma_alloc() does not
> support that attribute and rejects the allocation, causing DMA
> allocations to fail.
> 
> Do not force DMA allocations to be unencrypted when the device is using
> the IOMMU. This allows the IOMMU to map the encrypted physical pages as
> before and avoids incorrectly requesting CC_SHARED allocations.
> 
> Fixes: 8277a12d0d60 ("dma-pool: track decrypted atomic pools and select them via attrs")
> Reported-by: Timo Witte <timo.witte@gmail.com>
> Link: https://lore.kernel.org/all/CANB4YXR7h8V5Xp=MXVZeSdvw9UiriSagp=E+ju5RRDNghoPHLQ@mail.gmail.com
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
> ---
>   arch/x86/mm/mem_encrypt.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index 912f22ca838f..a349d8d21569 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -13,6 +13,7 @@
>   #include <linux/cc_platform.h>
>   #include <linux/mem_encrypt.h>
>   #include <linux/virtio_anchor.h>
> +#include <linux/iommu-dma.h>
>   
>   #include <asm/sev.h>
>   
> @@ -30,7 +31,7 @@ bool force_dma_unencrypted(struct device *dev)
>   	 * device does not support DMA to addresses that include the
>   	 * encryption mask.
>   	 */
> -	if (cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT)) {
> +	if (cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT) && !use_dma_iommu(dev)) {
>   		u64 dma_enc_mask = DMA_BIT_MASK(__ffs64(sme_me_mask));
>   		u64 dma_dev_mask = min_not_zero(dev->coherent_dma_mask,
>   						dev->bus_dma_limit);

if we drop this whole "if (cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT))" chunk, won't dma_alloc_attrs()&co be able to figure out this from dev->bus_dma_limit and other DMA masks we have all over the place? I should probably just try it... Thanks,


-- 
Alexey