lkml.org 
[lkml]   [2017]   [Feb]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [RFC PATCH v4 01/28] x86: Documentation for AMD Secure Memory Encryption (SME)
    From
    Date
    On 02/16/2017 11:56 AM, Borislav Petkov wrote:
    > Ok, this time detailed review :-)
    >
    > On Thu, Feb 16, 2017 at 09:42:11AM -0600, Tom Lendacky wrote:
    >> This patch adds a Documenation entry to decribe the AMD Secure Memory
    >> Encryption (SME) feature.
    >
    > Please introduce a spellchecker into your patch creation workflow. I see
    > two typos in one line.
    >
    > Also, never start patch commit messages with "This patch" - we know it
    > is this patch. Always write a doer-sentences explaining the why, not the
    > what. Something like:
    >
    > "Add a SME and mem_encrypt= kernel parameter documentation."
    >
    > for example.

    Ok, will do.

    >
    >> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
    >> ---
    >> 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
    >
    > "Refer to Documentation/x86/amd-memory-encryption.txt .."

    Ok.

    >
    >> + 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
    >
    > "... how to determine its position)."

    Ok.

    >
    >> +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
    >
    > s/Bit/Bits/

    Ok.

    >
    >> + Bit[11:6] reduction in physical address space, in bits, when
    >
    > Ditto.
    >
    >> + 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
    >
    > Let's use the kernel's define name MSR_K8_SYSCFG to avoid ambiguity.

    Will do.

    >
    >> +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.
    >
    > Ditto.
    >
    >> +
    >> + - 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.
    >
    > "... This is done by supplying mem_encrypt=on on the kernel command line.
    > Alternatively, if the kernel should enable SME by default, set
    > CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT=y."

    Yup, much clearer.

    >
    >> However, if BIOS does not enable SME, then Linux will not
    >> +attempt to activate memory encryption, even if configured to do so by default
    >
    > will not attempt or will not be able to?

    Probably closer to will not be able to right now. I'll update that.

    Thanks,
    Tom

    >
    >> +or the mem_encrypt=on command line parameter is specified.
    >

    \
     
     \ /
      Last update: 2017-02-16 20:49    [W:3.900 / U:0.004 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site