arm64: realm: Use aliased addresses for device DMA to shared buffers

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_addr_{encrypted, canonical}
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 <jean-philippe@linaro.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
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>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: 42be24a ("arm64: Enable memory encrypt for Realms")
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250227144150.1667735-4-suzuki.poulose@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/mem_encrypt.h

@@ -21,4 +21,15 @@ static inline bool force_dma_unencrypted(struct device *dev)
 	return is_realm_world();
 }
 
+/*
+ * For Arm CCA guests, canonical addresses are "encrypted", so no changes
+ * required for dma_addr_encrypted().
+ * The unencrypted DMA buffers must be accessed via the unprotected IPA,
+ * "top IPA bit" set.
+ */
+#define dma_addr_unencrypted(x)		((x) | PROT_NS_SHARED)
+
+/* Clear the "top" IPA bit while converting back */
+#define dma_addr_canonical(x)		((x) & ~PROT_NS_SHARED)
+
 #endif	/* __ASM_MEM_ENCRYPT_H */