[PATCH v7 14/15] Docs/x86/sgx: Add description for cgroup support

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From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

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

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Co-developed-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
Co-developed-by: Haitao Huang<haitao.huang@xxxxxxxxxxxxxxx>
Signed-off-by: Haitao Huang<haitao.huang@xxxxxxxxxxxxxxx>
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
- 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

- 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 | 74 ++++++++++++++++++++++++++++++++++
 1 file changed, 74 insertions(+)

diff --git a/Documentation/arch/x86/sgx.rst b/Documentation/arch/x86/sgx.rst
index d90796adc2ec..dfc8fac13ab2 100644
--- a/Documentation/arch/x86/sgx.rst
+++ b/Documentation/arch/x86/sgx.rst
@@ -300,3 +300,77 @@ 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
+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.
+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.

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