[PATCH v2 18/18] Docs/x86/sgx: Add description for cgroup support

[Kristen Carlson Accardi <kristen@linux.intel.com>]

Add initial documentation of how to regulate the distribution of
SGX Enclave Page Cache (EPC) memory via the Miscellaneous cgroup
controller.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Cc: Sean Christopherson <seanjc@google.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
---
 Documentation/x86/sgx.rst | 77 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)

diff --git a/Documentation/x86/sgx.rst b/Documentation/x86/sgx.rst
index 2bcbffacbed5..f6ca5594dcf2 100644
--- a/Documentation/x86/sgx.rst
+++ b/Documentation/x86/sgx.rst
@@ -300,3 +300,80 @@ to expected failures and handle them as follows:
    first call.  It indicates a bug in the kernel or the userspace client
    if any of the second round of ``SGX_IOC_VEPC_REMOVE_ALL`` calls has
    a return code other than 0.
+
+
+Cgroup Support
+==============
+
+The "sgx_epc" resource within the Miscellaneous cgroup controller regulates
+distribution of SGX EPC memory, which is a subset of system RAM that
+is used to provide SGX-enabled applications with protected memory,
+and is otherwise inaccessible, i.e. shows up as reserved in
+/proc/iomem and cannot be read/written outside of an SGX enclave.
+
+Although current systems implement EPC by stealing memory from RAM,
+for all intents and purposes the EPC is independent from normal system
+memory, e.g. must be reserved at boot from RAM and cannot be converted
+between EPC and normal memory while the system is running.  The EPC is
+managed by the SGX subsystem and is not accounted by the memory
+controller.  Note that this is true only for EPC memory itself, i.e.
+normal memory allocations related to SGX and EPC memory, e.g. the
+backing memory for evicted EPC pages, are accounted, limited and
+protected by the memory controller.
+
+Much like normal system memory, EPC memory can be overcommitted via
+virtual memory techniques and pages can be swapped out of the EPC
+to their backing store (normal system memory allocated via shmem).
+The SGX EPC subsystem is analogous to the memory subsytem, and
+it implements limit and protection models for EPC memory.
+
+SGX EPC Interface Files
+-----------------------
+
+For a generic description of the Miscellaneous controller interface
+files, please see Documentation/admin-guide/cgroup-v2.rst
+
+All SGX EPC memory amounts are in bytes unless explicitly stated
+otherwise.  If a value which is not PAGE_SIZE aligned is written,
+the actual value used by the controller will be rounded down to
+the closest PAGE_SIZE multiple.
+
+  misc.capacity
+        A read-only flat-keyed file shown only in the root cgroup.
+        The sgx_epc resource will show the total amount of EPC
+        memory available on the platform.
+
+  misc.current
+        A read-only flat-keyed file shown in the non-root cgroups.
+        The sgx_epc resource will show the current active EPC memory
+        usage of the cgroup and its descendants. EPC pages that are
+        swapped out to backing RAM are not included in the current count.
+
+  misc.max
+        A read-write single value file which exists on non-root
+        cgroups. The sgx_epc resource will show the EPC usage
+        hard limit. The default is "max".
+
+        If a cgroup's EPC usage reaches this limit, EPC allocations,
+        e.g. for page fault handling, will be blocked until EPC can
+        be reclaimed from the cgroup.  If EPC cannot be reclaimed in
+        a timely manner, reclaim will be forced, e.g. by ignoring LRU.
+
+  misc.events
+	A read-write flat-keyed file which exists on non-root cgroups.
+	Writes to the file reset the event counters to zero.  A value
+	change in this file generates a file modified event.
+
+	  max
+		The number of times the cgroup has triggered a reclaim
+		due to its EPC usage approaching (or exceeding) its max
+		EPC boundary.
+
+Migration
+---------
+
+Once an EPC page is charged to a cgroup (during allocation), it
+remains charged to the original cgroup until the page is released
+or reclaimed.  Migrating a process to a different cgroup doesn't
+move the EPC charges that it incurred while in the previous cgroup
+to its new cgroup.
-- 
2.38.1