PRM vs EPC

[Meysam_t_]

What is the difference between PRM (Processor Reserved Memory) and EPC (Enclave Page Cache) ?

I know that EPC is a subset of PRM and PRM is not visible to OS or VM but If entire PRM is not covered by EPC why that space is empty? and if it is fully covered by EPC why we put another name on it?

[yunfeng7854]

There are other metadata about the EPC pages, such as the EPCM, stored in the PRM. So that space is not empty.

[Meysam_t_]

Thanks for your answer,

How does CPU protect the PRM and EPC? Is there any difference in the way they are protected? how does CPU make sure that PRM or EPC is invisible to OS and other enclaves?

[Shivananda_H_Intel]

There are specific SGX CPU Instructions for managing the EPC Pages. You can refer to Page 41 Section  3.5.2 OS Management of EPC Pages in below document.

https://software.intel.com/sites/default/files/managed/48/88/329298-002.pdf

Regards

Shivananda