Re: [PATCH v2 3/3] arm64: realm: Use aliased addresses for device DMA to shared buffers

Hi Gavin

Thanks for the review.

On 25/02/2025 05:28, Gavin Shan wrote:

On 2/25/25 3:24 PM, Gavin Shan wrote:

quoted

On 2/20/25 8:07 AM, Suzuki K Poulose wrote:

quoted

When a device performs DMA to a shared buffer using physical addresses,
(without Stage1 translation), the device must use the "{I}PA address" 
with the
top bit set in Realm. This is to make sure that a trusted device will 
be able
to write to shared buffers as well as the protected buffers. Thus, a 
Realm must
always program the full address including the "protection" bit, like 
AMD SME
encryption bits.

Enable this by providing arm64 specific 
dma_{encrypted,decrypted,clear_encryption}
helpers for Realms. Please note that the VMM needs to similarly make 
sure that
the SMMU Stage2 in the Non-secure world is setup accordingly to map 
IPA at the
unprotected alias.

Cc: Will Deacon <will@kernel.org>
Cc: Jean-Philippe Brucker <redacted>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
---
  arch/arm64/include/asm/mem_encrypt.h | 22 ++++++++++++++++++++++
  1 file changed, 22 insertions(+)

diff --git a/arch/arm64/include/asm/mem_encrypt.h b/arch/arm64/ 

include/asm/mem_encrypt.h
index f8f78f622dd2..aeda3bba255e 100644

--- a/arch/arm64/include/asm/mem_encrypt.h
+++ b/arch/arm64/include/asm/mem_encrypt.h

@@ -21,4 +21,26 @@ static inline bool force_dma_unencrypted(struct 

device *dev)
      return is_realm_world();
  }
+static inline dma_addr_t dma_decrypted(dma_addr_t daddr)
+{
+    if (is_realm_world())
+        daddr |= prot_ns_shared;
+    return daddr;
+}
+#define dma_decrypted dma_decrypted
+

There is an existing macro (PROT_NS_SHARED), which is preferred to return
prot_ns_shared or 0 depending on the availability of the realm 
capability.
However, that macro needs to be improved a bit so that it can be used 
here.
We need to return 0UL to match with the type of prot_ns_shared 
(unsigned long)

-#define PROT_NS_SHARED         (is_realm_world() ? prot_ns_shared : 0)
+#define PROT_NS_SHARED         (is_realm_world() ? prot_ns_shared : 0UL)

After that, the chunk of code can be as below.

     return daddr | PROT_NS_SHARED;

quoted

+static inline dma_addr_t dma_encrypted(dma_addr_t daddr)
+{
+    if (is_realm_world())
+        daddr &= prot_ns_shared - 1;
+    return daddr;
+}
+#define dma_encrypted dma_encrypted
+

With PROT_NS_SHARED, it can become something like below. 
(PROT_NS_SHARED - 1)
is equivalent to -1UL, 'daddr & -1UL' should be fine since it does 
nothing.

I meant (PROT_NS_SHARED - 1) is equivalent to -1UL when no realm capability
is around :)

I didn't want this to be there ;-). But with Robin's comment, I think we
can revert back to PROT_NS_SHARED.

Cheers
Suzuki

quoted

     return daddr & (PROT_NS_SHARED - 1);

quoted

+static inline dma_addr_t dma_clear_encryption(dma_addr_t daddr)
+{
+    return dma_encrypted(daddr);
+}
+#define dma_clear_encryption dma_clear_encryption
+
  #endif    /* __ASM_MEM_ENCRYPT_H */

Thanks,
Gavin