[Xen-devel] [PATCH v9 01/38] x86: Document AMD Secure Memory Encryption (SME)

[Tom Lendacky <thomas.lendacky@xxxxxxx>]

Create a Documentation entry to describe the AMD Secure Memory
Encryption (SME) feature and add documentation for the mem_encrypt=
kernel parameter.

Reviewed-by: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt |   11 ++++
 Documentation/x86/amd-memory-encryption.txt     |   68 +++++++++++++++++++++++
 2 files changed, 79 insertions(+)
 create mode 100644 Documentation/x86/amd-memory-encryption.txt

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 7037a0f..05742cc 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2197,6 +2197,17 @@
                        memory contents and reserves bad memory
                        regions that are detected.
 
+       mem_encrypt=    [X86-64] AMD Secure Memory Encryption (SME) control
+                       Valid arguments: on, off
+                       Default (depends on kernel configuration option):
+                         on  (CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT=y)
+                         off (CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT=n)
+                       mem_encrypt=on:         Activate SME
+                       mem_encrypt=off:        Do not activate SME
+
+                       Refer to Documentation/x86/amd-memory-encryption.txt
+                       for details on when memory encryption can be activated.
+
        mem_sleep_default=      [SUSPEND] Default system suspend mode:
                        s2idle  - Suspend-To-Idle
                        shallow - Power-On Suspend or equivalent (if supported)
diff --git a/Documentation/x86/amd-memory-encryption.txt 
b/Documentation/x86/amd-memory-encryption.txt
new file mode 100644
index 0000000..f512ab7
--- /dev/null
+++ b/Documentation/x86/amd-memory-encryption.txt
@@ -0,0 +1,68 @@
+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 on how to determine its position).  The encryption bit can also be
+specified in the cr3 register, allowing the PGD table to be encrypted. Each
+successive level of page tables can also be encrypted by setting the encryption
+bit in the page table entry that points to the next table. This allows the full
+page table hierarchy to be encrypted. Note, this means that just because the
+encryption bit is set in cr3, doesn't imply the full hierarchy is encyrpted.
+Each page table entry in the hierarchy needs to have the encryption bit set to
+achieve that. So, theoretically, you could have the encryption bit set in cr3
+so that the PGD is encrypted, but not set the encryption bit in the PGD entry
+for a PUD which results in the PUD pointed to by that entry to not 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]:
+               Bits[5:0]  pagetable bit number used to activate memory
+                          encryption
+               Bits[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 (MSR_K8_SYSCFG) 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.
+
+The state of SME in the Linux kernel can be documented as follows:
+       - Supported:
+         The CPU supports SME (determined through CPUID instruction).
+
+       - Enabled:
+         Supported and bit 23 of MSR_K8_SYSCFG is set.
+
+       - Active:
+         Supported, Enabled and the Linux kernel is actively applying
+         the encryption bit to page table entries (the SME mask in the
+         kernel is non-zero).
+
+SME can also be enabled and activated in the BIOS. If SME is enabled and
+activated in the BIOS, then all memory accesses will be encrypted and it will
+not be necessary to activate the Linux memory encryption support.  If the BIOS
+merely enables SME (sets bit 23 of the MSR_K8_SYSCFG), then Linux can activate
+memory encryption by default (CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT=y) or
+by supplying mem_encrypt=on on the kernel command line.  However, if BIOS does
+not enable SME, then Linux will not be able to activate memory encryption, even
+if configured to do so by default or the mem_encrypt=on command line parameter
+is specified.


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