x86/mm: Move force_dma_unencrypted() to common code

Intel TDX doesn't allow VMM to access guest private memory. Any memory
that is required for communication with VMM must be shared explicitly
by setting the bit in page table entry. After setting the shared bit,
the conversion must be completed with MapGPA hypercall. Details about
MapGPA hypercall can be found in [1], sec 3.2.

The call informs VMM about the conversion between private/shared
mappings. The shared memory is similar to unencrypted memory in AMD
SME/SEV terminology but the underlying process of sharing/un-sharing
the memory is different for Intel TDX guest platform.

SEV assumes that I/O devices can only do DMA to "decrypted" physical
addresses without the C-bit set. In order for the CPU to interact with
this memory, the CPU needs a decrypted mapping. To add this support,
AMD SME code forces force_dma_unencrypted() to return true for
platforms that support AMD SEV feature. It will be used for DMA memory
allocation API to trigger set_memory_decrypted() for platforms that
support AMD SEV feature.

TDX is similar. So, to communicate with I/O devices, related pages need
to be marked as shared. As mentioned above, shared memory in TDX
architecture is similar to decrypted memory in AMD SME/SEV. So similar
to AMD SEV, force_dma_unencrypted() has to forced to return true. This
support is added in other patches in this series.

So move force_dma_unencrypted() out of AMD specific code and call AMD
specific (amd_force_dma_unencrypted()) initialization function from it.
force_dma_unencrypted() will be modified by later patches to include
Intel TDX guest platform specific initialization.

Also, introduce new config option X86_MEM_ENCRYPT_COMMON that has to be
selected by all x86 memory encryption features. This will be selected
by both AMD SEV and Intel TDX guest config options.

This is preparation for TDX changes in DMA code and it has no
functional change.

[1] - https://software.intel.com/content/dam/develop/external/us/en/documents/intel-tdx-guest-hypervisor-communication-interface.pdf

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>

arch/x86/Kconfig

@@ -1524,16 +1524,20 @@ config X86_CPA_STATISTICS
 	  helps to determine the effectiveness of preserving large and huge
 	  page mappings when mapping protections are changed.
 
+config X86_MEM_ENCRYPT_COMMON
+	select ARCH_HAS_FORCE_DMA_UNENCRYPTED
+	select DYNAMIC_PHYSICAL_MASK
+	def_bool n
+
 config AMD_MEM_ENCRYPT
 	bool "AMD Secure Memory Encryption (SME) support"
 	depends on X86_64 && CPU_SUP_AMD
 	select DMA_COHERENT_POOL
-	select DYNAMIC_PHYSICAL_MASK
 	select ARCH_USE_MEMREMAP_PROT
-	select ARCH_HAS_FORCE_DMA_UNENCRYPTED
 	select INSTRUCTION_DECODER
 	select ARCH_HAS_RESTRICTED_VIRTIO_MEMORY_ACCESS
 	select ARCH_HAS_CC_PLATFORM
+	select X86_MEM_ENCRYPT_COMMON
 	help
 	  Say yes to enable support for the encryption of system memory.
 	  This requires an AMD processor that supports Secure Memory

arch/x86/include/asm/mem_encrypt_common.h

@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (C) 2020 Intel Corporation */
+#ifndef _ASM_X86_MEM_ENCRYPT_COMMON_H
+#define _ASM_X86_MEM_ENCRYPT_COMMON_H
+
+#include <linux/mem_encrypt.h>
+#include <linux/device.h>
+
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+bool amd_force_dma_unencrypted(struct device *dev);
+#else /* CONFIG_AMD_MEM_ENCRYPT */
+static inline bool amd_force_dma_unencrypted(struct device *dev)
+{
+	return false;
+}
+#endif /* CONFIG_AMD_MEM_ENCRYPT */
+
+#endif

arch/x86/mm/Makefile

@@ -52,6 +52,8 @@ obj-$(CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS)	+= pkeys.o
 obj-$(CONFIG_RANDOMIZE_MEMORY)			+= kaslr.o
 obj-$(CONFIG_PAGE_TABLE_ISOLATION)		+= pti.o
 
+obj-$(CONFIG_X86_MEM_ENCRYPT_COMMON)	+= mem_encrypt_common.o
+
 obj-$(CONFIG_AMD_MEM_ENCRYPT)	+= mem_encrypt.o
 obj-$(CONFIG_AMD_MEM_ENCRYPT)	+= mem_encrypt_identity.o
 obj-$(CONFIG_AMD_MEM_ENCRYPT)	+= mem_encrypt_boot.o

arch/x86/mm/mem_encrypt.c

@@ -31,6 +31,7 @@
 #include <asm/processor-flags.h>
 #include <asm/msr.h>
 #include <asm/cmdline.h>
+#include <asm/mem_encrypt_common.h>
 
 #include "mm_internal.h"
 
@@ -362,7 +363,7 @@ int __init early_set_memory_encrypted(unsigned long vaddr, unsigned long size)
 }
 
 /* Override for DMA direct allocation check - ARCH_HAS_FORCE_DMA_UNENCRYPTED */
-bool force_dma_unencrypted(struct device *dev)
+bool amd_force_dma_unencrypted(struct device *dev)
 {
 	/*
 	 * For SEV, all DMA must be to unencrypted addresses.

arch/x86/mm/mem_encrypt_common.c

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Memory Encryption Support Common Code
+ *
+ * Copyright (C) 2021 Intel Corporation
+ *
+ * Author: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
+ */
+
+#include <asm/mem_encrypt_common.h>
+#include <linux/dma-mapping.h>
+
+/* Override for DMA direct allocation check - ARCH_HAS_FORCE_DMA_UNENCRYPTED */
+bool force_dma_unencrypted(struct device *dev)
+{
+	return amd_force_dma_unencrypted(dev);
+}