Re: [RFC PATCH v2 01/20] x86: Documentation for AMD Secure Memory Encryption (SME)

[Borislav Petkov]

On Mon, Aug 22, 2016 at 05:35:39PM -0500, 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/x86/amd-memory-encryption.txt |   35 +++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
>  create mode 100644 Documentation/x86/amd-memory-encryption.txt
> 
> diff --git a/Documentation/x86/amd-memory-encryption.txt b/Documentation/x86/amd-memory-encryption.txt
> new file mode 100644
> index 0000000..f19c555
> --- /dev/null
> +++ b/Documentation/x86/amd-memory-encryption.txt
> @@ -0,0 +1,35 @@
> +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 encrpyted will be

s/encrpyted/encrypted/

> +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.
> +
> +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 in the kernel through the AMD_MEM_ENCRYPT config
> +option.

" ... is configurable through CONFIG_AMD_MEM_ENCRYPT."

> Additionally, the mem_encrypt=on command line parameter is required
> +to activate memory encryption.

I think you want to rewrite the logic here to say that people should use
the BIOS option and if none is present for whatever reason, resort to
the alternative "mem_encrypt=on" kernel command line option, no?

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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