Re: [PATCH 1/1] arm64: realm: Use aliased addresses for device DMA to shared buffers

On 15/02/2025 14:38, Aneesh Kumar K.V wrote:

Aneesh Kumar K.V [off-list ref] writes:

quoted

Suzuki K Poulose [off-list ref] writes:

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.

Add the support for this by providing arm64 version of the phys_to_dma(). We
cannot use the __sme_mask as it assumes the "encryption" always "sets a bit",
which is the opposite for CCA. i.e., "set a bit" for "decrypted" address. So,
move the common code that can be reused by all - i.e., add __phys_to_dma() and
__dma_to_phys() - and do the arch specific processing.

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>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
---
  arch/arm64/Kconfig                  |  1 +
  arch/arm64/include/asm/dma-direct.h | 38 +++++++++++++++++++++++++++++
  include/linux/dma-direct.h          | 35 +++++++++++++++++---------
  3 files changed, 62 insertions(+), 12 deletions(-)
  create mode 100644 arch/arm64/include/asm/dma-direct.h

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index fcdd0ed3eca8..7befe04106de 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -41,6 +41,7 @@ config ARM64
  	select ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
  	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
  	select ARCH_HAS_NONLEAF_PMD_YOUNG if ARM64_HAFT
+	select ARCH_HAS_PHYS_TO_DMA
  	select ARCH_HAS_PTE_DEVMAP
  	select ARCH_HAS_PTE_SPECIAL
  	select ARCH_HAS_HW_PTE_YOUNG

diff --git a/arch/arm64/include/asm/dma-direct.h b/arch/arm64/include/asm/dma-direct.h
new file mode 100644
index 000000000000..37c3270542b8
--- /dev/null
+++ b/arch/arm64/include/asm/dma-direct.h

@@ -0,0 +1,38 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef __ASM_DMA_DIRECT_H
+#define __ASM_DMA_DIRECT_H
+
+#include <asm/pgtable-prot.h>
+
+static inline unsigned long addr_to_shared(unsigned long addr)
+{
+	if (is_realm_world())
+		addr |= prot_ns_shared;
+	return addr;
+}
+
+static inline unsigned long addr_to_private(unsigned long addr)
+{
+	if (is_realm_world())
+		addr &= prot_ns_shared - 1;
+	return addr;
+}
+
+static inline dma_addr_t phys_to_dma(struct device *dev, phys_addr_t paddr)
+{
+	return __phys_to_dma(dev, paddr);
+}
+
+static inline dma_addr_t phys_to_dma_unencrypted(struct device *dev,
+						 phys_addr_t paddr)
+{
+	return addr_to_shared(__phys_to_dma(dev, paddr));
+}
+#define phys_to_dma_unencrypted phys_to_dma_unencrypted
+
+static inline phys_addr_t dma_to_phys(struct device *dev, dma_addr_t dma_addr)
+{
+	return addr_to_private(__dma_to_phys(dev, dma_addr));
+}
+
+#endif	/* __ASM_DMA_DIRECT_H */

diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
index d7e30d4f7503..3e9bf6ca640e 100644
--- a/include/linux/dma-direct.h
+++ b/include/linux/dma-direct.h

@@ -72,18 +72,36 @@ static inline dma_addr_t dma_range_map_max(const struct bus_dma_region *map)
  	return ret;
  }
  
+static inline dma_addr_t __phys_to_dma(struct device *dev, phys_addr_t paddr)
+{
+	if (dev->dma_range_map)
+		return translate_phys_to_dma(dev, paddr);
+	return paddr;
+}
+
+static inline phys_addr_t __dma_to_phys(struct device *dev, dma_addr_t dma_addr)
+{
+	phys_addr_t paddr;
+
+	if (dev->dma_range_map)
+		paddr = translate_dma_to_phys(dev, dma_addr);
+	else
+		paddr = dma_addr;
+
+	return paddr;
+}
+
  #ifdef CONFIG_ARCH_HAS_PHYS_TO_DMA
  #include <asm/dma-direct.h>
  #ifndef phys_to_dma_unencrypted
  #define phys_to_dma_unencrypted		phys_to_dma
  #endif
  #else
+
  static inline dma_addr_t phys_to_dma_unencrypted(struct device *dev,
  		phys_addr_t paddr)
  {
-	if (dev->dma_range_map)
-		return translate_phys_to_dma(dev, paddr);
-	return paddr;
+	return __phys_to_dma(dev, paddr);
  }
  
  /*

@@ -94,19 +112,12 @@ static inline dma_addr_t phys_to_dma_unencrypted(struct device *dev,
   */
  static inline dma_addr_t phys_to_dma(struct device *dev, phys_addr_t paddr)
  {
-	return __sme_set(phys_to_dma_unencrypted(dev, paddr));
+	return __sme_set(__phys_to_dma(dev, paddr));
  }
  
  static inline phys_addr_t dma_to_phys(struct device *dev, dma_addr_t dma_addr)
  {
-	phys_addr_t paddr;
-
-	if (dev->dma_range_map)
-		paddr = translate_dma_to_phys(dev, dma_addr);
-	else
-		paddr = dma_addr;
-
-	return __sme_clr(paddr);
+	return __sme_clr(__dma_to_phys(dev, dma_addr));
  }
  #endif /* !CONFIG_ARCH_HAS_PHYS_TO_DMA */
  

-- 
2.43.0

How about the below?

The function name addr_to_shared is too generic to be included in the
dma-direct.h header file. Since we don’t expect it to be called
directly, we can either inline it or find a more specific name.

Additionally, for dma_to_phys conversion, we first retrieve the private
address/alias before switching to the physical address. While both
approaches yield the correct result, this change more clearly defines the
conversion rules?

Also translate_dma_to_phys/translate_phys_to_dma work with private dma
addr/alias right?

You're right, that needs to be fixed. Luckily (hopefully) DMA ranges
are not used by AMD SME.

Suzuki

quoted

modified   arch/arm64/include/asm/dma-direct.h

@@ -4,14 +4,14 @@
  
  #include <asm/pgtable-prot.h>
  
-static inline unsigned long addr_to_shared(unsigned long addr)
+static inline unsigned long shared_dma_addr(unsigned long addr)
  {
  	if (is_realm_world())
  		addr |= prot_ns_shared;
  	return addr;
  }
  
-static inline unsigned long addr_to_private(unsigned long addr)
+static inline unsigned long private_dma_addr(unsigned long addr)
  {
  	if (is_realm_world())
  		addr &= prot_ns_shared - 1;

@@ -26,13 +26,14 @@ static inline dma_addr_t phys_to_dma(struct device *dev, phys_addr_t paddr)
  static inline dma_addr_t phys_to_dma_unencrypted(struct device *dev,
  						 phys_addr_t paddr)
  {
-	return addr_to_shared(__phys_to_dma(dev, paddr));
+	return shared_dma_addr(__phys_to_dma(dev, paddr));
  }
  #define phys_to_dma_unencrypted phys_to_dma_unencrypted
  
  static inline phys_addr_t dma_to_phys(struct device *dev, dma_addr_t dma_addr)
  {
-	return addr_to_private(__dma_to_phys(dev, dma_addr));
+	/* if it is a shared dma addr convert to private before looking for phys_addr */
+	return __dma_to_phys(dev, private_dma_addr(dma_addr));
  }
  
  #endif	/* __ASM_DMA_DIRECT_H */