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

[Tom Lendacky]

On 11/10/2016 4:51 AM, Borislav Petkov wrote:
> On Wed, Nov 09, 2016 at 06:34:39PM -0600, Tom Lendacky wrote:
>> 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.
> 
> I'd say here:
> 
> 			"Force-enable memory encryption if it is disabled in the
> 			BIOS."

Good suggestion, that will make this clearer.

> 
>> +			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.
> 
> So how am I to understand this? We won't have TSME or we will but it
> will be off by default and users will have to enable it in the BIOS or
> will have to boot with mem_encrypt=on...?
> 
> Can you please expand on all the possible options there would be
> available to users?

Yup, I'll try to expand on the documentation to include all the
possibilities for this.

Thanks,
Tom

>