[PATCH for_v21] docs: x86/sgx: Add a blurb on basic EPC management to 'kernel internals'

[Sean Christopherson <sean.j.christopherson@intel.com>]

Add a section on how EPC is managed and why.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 Documentation/x86/sgx/2.Kernel-internals.rst | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/Documentation/x86/sgx/2.Kernel-internals.rst b/Documentation/x86/sgx/2.Kernel-internals.rst
index de359bf605ca..5c90a65936f2 100644
--- a/Documentation/x86/sgx/2.Kernel-internals.rst
+++ b/Documentation/x86/sgx/2.Kernel-internals.rst
@@ -15,6 +15,26 @@ attempt to play nice in the face of a misconfigured system.  With the exception
 of Launch Control's hash MSRs, which can vary per CPU, Linux assumes that all
 CPUs have a configuration that is identical to the boot CPU.
 
+EPC management
+==============
+
+Because the kernel can't arbitrarily read EPC memory or share RO backing pages
+between enclaves, traditional memory models such as CoW and fork() do not work
+with enclaves.  In other words, the architectural rules of EPC forces it to be
+treated as MAP_SHARED at all times.
+
+The inability to employ traditional memory models also means that EPC memory
+must be isolated from normal memory pools, e.g. attempting to use EPC memory
+for normal mappings would result in faults and/or perceived data corruption.
+Furthermore, EPC is not enumerated by as normal memory, e.g. BIOS enumerates
+EPC as reserved memory in the e820 tables, or not at all.  As a result, EPC
+memory is directly managed by the SGX subsystem, e.g. SGX employs VM_PFNMAP to
+manually insert/zap/swap page table entries, and exposes EPC to userspace via
+a well known device, /dev/sgx/enclave.
+
+The net effect is that all enclave VMAs must be MAP_SHARED and are backed by
+a single file, /dev/sgx/enclave.
+
 EPC oversubscription
 ====================
 
-- 
2.22.0