[RFC PATCH 1/2] dma-mapping: Avoid double decrypting with DMA_RESTRICTED_POOL

[Mostafa Saleh]

In case a device have a restricted DMA pool, it will be decrypted.
However, in the path of dma_direct_alloc() memory can be allocated
from this pool using, __dma_direct_alloc_pages() =>
dma_direct_alloc_swiotlb()

After that from the same function, it will attempt to decrypt it
using dma_set_decrypted() if force_dma_unencrypted().

Which results in the memory being decrypted twice.

It's not clear how the does realm world/hypervisors deal with that,
for example:
- Clear a bit in the page table and call realm IPA_STATE_SET
- TDX: Seems to issue a hypercall also.
- pKVM: Which doesn't implement force_dma_unencrypted() at the moment,
  uses a share hypercall which is definitely not Idempotent.

This patch will only encrypt/decrypt memory that are not allocated
form the restricted dma pools.

Signed-off-by: Mostafa Saleh <smostafa@xxxxxxxxxx>
---
 kernel/dma/direct.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 8f43a930716d..27d804f0473f 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -79,7 +79,7 @@ bool dma_coherent_ok(struct device *dev, phys_addr_t phys, size_t size)
 
 static int dma_set_decrypted(struct device *dev, void *vaddr, size_t size)
 {
-	if (!force_dma_unencrypted(dev))
+	if (!force_dma_unencrypted(dev) || is_swiotlb_for_alloc(dev))
 		return 0;
 	return set_memory_decrypted((unsigned long)vaddr, PFN_UP(size));
 }
@@ -88,7 +88,7 @@ static int dma_set_encrypted(struct device *dev, void *vaddr, size_t size)
 {
 	int ret;
 
-	if (!force_dma_unencrypted(dev))
+	if (!force_dma_unencrypted(dev) || is_swiotlb_for_alloc(dev))
 		return 0;
 	ret = set_memory_encrypted((unsigned long)vaddr, PFN_UP(size));
 	if (ret)
-- 
2.53.0.473.g4a7958ca14-goog