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WGall3
Beginner
708 Views

Understanding MaxEnclaveSize

Hi,

I am working with the SGX Enclave on Ubuntu 18.04 with an HP g5 Laptop with i5-6200U CPU. I have installed the linux driver and successfully built the SDK and run the sample enclave programs. However, I had a question regarding the size of the enclaves. I believe the term for the memory reserved for the Enclave is known as Processor Reserved Memory (PRM) and this is set in the BIOS. However, when I enabled SGX in the BIOS I saw only three options: Enabled, Disabled, and Software Controlled. I did see an option for changing the amount of PRM. Have the manufacturers not provided this option?

I assume that the PRM is 128 MB as this is the maximum size but how can I confirm this? Running the code found here

https://github.com/ayeks/SGX-hardware/blob/master/test-sgx.c

Gives the results:

MaxEnclaveSize_Not64: 1f
MaxEnclaveSize_64: 24

What do these numbers mean?

Thanks,

Will

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708 Views

Those results refer to the values returned by the CPUID call (Section 37.7.2 of https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-vol-3d-part-4-manual.pdf )

MaxEnclaveSize_Not64: the maximum supported enclave size is 2^(EDX[7:0]) bytes when not in 64-bit mode
MaxEnclaveSize_64: the maximum supported enclave size is 2^(EDX[15:8]) bytes when operating in 64- bit mode.

So the maximum Virtual Size of the enclave is 2^(0x1f) for 32bit and 2^(0x24) for 64bit enclaves.

For how much EPC, use Table 37-6. From the test program you linked, the line you are looking for is:

CPUID Leaf 12H, Sub-Leaf 2 of Intel SGX Capabilities (EAX=12H,ECX=2)
eax: 80000001 ebx: 0 ecx: 2000001 edx: 0

On my system 
0x2000000 matches the 32 MB I set up on this particular system. On a 128 MB system, you should get 0 for edx and for ecx:
0x‭800000#  (and you can ignore the last hex value for the purposes of determining the size of EPC)

Hope this helps.

Francisco

WGall3
Beginner
708 Views

Francisco,

Thank you for your reply. It answers my question quite clearly.

-Will

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