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    Subject[PATCH v5 01/32] x86: Documentation for AMD Secure Memory Encryption (SME)
    Date
    Create a Documentation entry to describe the AMD Secure Memory
    Encryption (SME) feature and add documentation for the mem_encrypt=
    kernel parameter.

    Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
    ---
    Documentation/admin-guide/kernel-parameters.txt | 11 ++++
    Documentation/x86/amd-memory-encryption.txt | 60 +++++++++++++++++++++++
    2 files changed, 71 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 3dd6d5d..84c5787 100644
    --- a/Documentation/admin-guide/kernel-parameters.txt
    +++ b/Documentation/admin-guide/kernel-parameters.txt
    @@ -2165,6 +2165,17 @@
    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..0b72ff2
    --- /dev/null
    +++ b/Documentation/x86/amd-memory-encryption.txt
    @@ -0,0 +1,60 @@
    +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 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]:
    + 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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