[RFC PATCH v3 01/20] x86: Documentation for AMD Secure Memory Encryption (SME)

[Tom Lendacky]

This patch adds a Documenation entry to decribe the AMD Secure Memory
Encryption (SME) feature.

Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---
 Documentation/kernel-parameters.txt         |    5 +++
 Documentation/x86/amd-memory-encryption.txt |   40 +++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)
 create mode 100644 Documentation/x86/amd-memory-encryption.txt

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 030e9e9..4c730b0 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2282,6 +2282,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			memory contents and reserves bad memory
 			regions that are detected.
 
+	mem_encrypt=	[X86-64] Enable AMD Secure Memory Encryption (SME)
+			Memory encryption is disabled by default, using this
+			switch, memory encryption can be enabled.
+			on: enable memory encryption
+
 	meye.*=		[HW] Set MotionEye Camera parameters
 			See Documentation/video4linux/meye.txt.
 
diff --git a/Documentation/x86/amd-memory-encryption.txt b/Documentation/x86/amd-memory-encryption.txt
new file mode 100644
index 0000000..788d871
--- /dev/null
+++ b/Documentation/x86/amd-memory-encryption.txt
@@ -0,0 +1,40 @@
+Secure Memory Encryption (SME) is a feature found on AMD processors.
+
+SME provides the ability to mark individual pages of memory as encrypted using
+the standard x86 page tables.  A page that is marked encrypted will be
+automatically decrypted when read from DRAM and encrypted when written to
+DRAM.  SME can therefore be used to protect the contents of DRAM from physical
+attacks on the system.
+
+A page is encrypted when a page table entry has the encryption bit set (see
+below how to determine the position of the bit).  The encryption bit can be
+specified in the cr3 register, allowing the PGD table to be encrypted. Each
+successive level of page tables can also be encrypted.
+
+Support for SME can be determined through the CPUID instruction. The CPUID
+function 0x8000001f reports information related to SME:
+
+	0x8000001f[eax]:
+		Bit[0] indicates support for SME
+	0x8000001f[ebx]:
+		Bit[5:0]  pagetable bit number used to enable memory encryption
+		Bit[11:6] reduction in physical address space, in bits, when
+			  memory encryption is enabled (this only affects system
+			  physical addresses, not guest physical addresses)
+
+If support for SME is present, MSR 0xc00100010 (SYS_CFG) can be used to
+determine if SME is enabled and/or to enable memory encryption:
+
+	0xc0010010:
+		Bit[23]   0 = memory encryption features are disabled
+			  1 = memory encryption features are enabled
+
+Linux relies on BIOS to set this bit if BIOS has determined that the reduction
+in the physical address space as a result of enabling memory encryption (see
+CPUID information above) will not conflict with the address space resource
+requirements for the system.  If this bit is not set upon Linux startup then
+Linux itself will not set it and memory encryption will not be possible.
+
+SME support is configurable through the AMD_MEM_ENCRYPT config option.
+Additionally, the mem_encrypt=on command line parameter is required to activate
+memory encryption.