[PATCH v8 14/15] Docs/x86/sgx: Add description for cgroup support

[Haitao Huang <haitao.huang@linux.intel.com>]

From: Sean Christopherson <sean.j.christopherson@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>
Co-developed-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Co-developed-by: Haitao Huang<haitao.huang@linux.intel.com>
Signed-off-by: Haitao Huang<haitao.huang@linux.intel.com>
Cc: Sean Christopherson <seanjc@google.com>
---
V8:
- Limit text width to 80 characters to be consistent.

V6:
- Remove mentioning of VMM specific behavior on handling SIGBUS
- Remove statement of forced reclamation, add statement to specify
ENOMEM returned when no reclamation possible.
- Added statements on the non-preemptive nature for the max limit
- Dropped Reviewed-by tag because of changes

V4:
- Fix indentation (Randy)
- Change misc.events file to be read-only
- Fix a typo for 'subsystem'
- Add behavior when VMM overcommit EPC with a cgroup (Mikko)
---
 Documentation/arch/x86/sgx.rst | 83 ++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)

diff --git a/Documentation/arch/x86/sgx.rst b/Documentation/arch/x86/sgx.rst
index d90796adc2ec..c537e6a9aa65 100644
--- a/Documentation/arch/x86/sgx.rst
+++ b/Documentation/arch/x86/sgx.rst
@@ -300,3 +300,86 @@ 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 subsystem, 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 there are no pages left that are reclaimable within the same
+        group, the kernel returns ENOMEM.
+
+        The EPC pages allocated for a guest VM by the virtual EPC driver are not
+        reclaimable by the host kernel. In case the guest cgroup's limit is
+        reached and no reclaimable pages left in the same cgroup, the virtual
+        EPC driver returns SIGBUS to the user space process to indicate failure
+        on new EPC allocation requests.
+
+        The misc.max limit is non-preemptive. If a user writes a limit lower
+        than the current usage to this file, the cgroup will not preemptively
+        deallocate pages currently in use, and will only start blocking the next
+        allocation and reclaiming EPC at that time.
+
+  misc.events
+        A read-only flat-keyed file which exists on non-root cgroups.
+        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.
-- 
2.25.1