Re: [BUG] iommu/dma: "Not yet supported" CC_SHARED rejection regresses dma_alloc_attrs() on SME hosts with IOMMU-attached GPUs (commit 8277a12d0d60)

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

Timo Witte <timo.witte@gmail.com> writes:

> Commit 8277a12d0d60 ("dma-pool: track decrypted atomic pools and select
> them via attrs") added to iommu_dma_alloc():
>
>     /* Not yet supported */
>     if (is_alloc_cc_shared)
>         return NULL;
>
> dma_alloc_attrs() auto-sets __DMA_ATTR_ALLOC_CC_SHARED whenever
> force_dma_unencrypted() is true. On an SME host (mem_encrypt=on) an AMD
> GPU with a 44-bit DMA mask hits that condition, so every GTT (system-RAM)
> allocation fails with NULL/-ENOMEM, amdgpu init aborts and the system
> boots without a display.
> IOMMU in Translated/DMA-FQ mode (iommu group 29).
>
> Before the mentioned patch it worked: the IOMMU PTE carries the SME C-bit,
> so the memory controller transparently decrypts for the device while pages
> stay
> encrypted at rest. For IOMMU-backed devices force_dma_unencrypted() is a
> false positive - the device DMA mask does not limit addressing once the
> IOMMU translates, so CC_SHARED should not be auto-set for them.
>


This means that, with host memory encryption, DMA does not need to be
forced unencrypted when the device uses the IOMMU?

modified   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);


-aneesh