This patch adds a Documenation entry to decribe the AMD Secure Memory Encryption (SME) feature. Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx> --- Documentation/admin-guide/kernel-parameters.txt | 11 ++++ Documentation/x86/amd-memory-encryption.txt | 57 +++++++++++++++++++++++ 2 files changed, 68 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 110745e..91c40fa 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -2145,6 +2145,17 @@ 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 the SME documentation 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..0938e89 --- /dev/null +++ b/Documentation/x86/amd-memory-encryption.txt @@ -0,0 +1,57 @@ +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 activate 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. + +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 the SYS_CFG MSR 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 SYS_CFG MSR), then Linux can activate +memory encryption. However, if BIOS does not enable SME, then Linux will not +attempt to activate memory encryption, even if configured to do so by default +or the mem_encrypt=on command line parameter is specified. -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
