sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 65 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-secvar b/Documentation/ABI/testing/sysfs-secvar
new file mode 100644
index 000000000..c52a5fd15
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-secvar
@@ -0,0 +1,65 @@
+What:		/sys/firmware/secvar
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	This directory is created if the POWER firmware supports OS
+		secureboot, thereby secure variables. It exposes interface
+		for reading/writing the secure variables
+
+What:		/sys/firmware/secvar/vars
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	This directory lists all the secure variables that are supported
+		by the firmware.
+
+What:		/sys/firmware/secvar/format
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	A string indicating which backend is in use by the firmware.
+		This determines the format of the variable and the accepted
+		format of variable updates.
+
+		On powernv/OPAL, this value is provided by the OPAL firmware
+		and is expected to be "ibm,edk2-compat-v1".
+
+		On pseries/PLPKS, this is generated by the kernel based on the
+		version number in the SB_VERSION variable in the keystore. The
+		version numbering in the SB_VERSION variable starts from 1. The
+		format string takes the form "ibm,plpks-sb-v<version>" in the
+		case of dynamic key management mode. If the SB_VERSION variable
+		does not exist (or there is an error while reading it), it takes
+		the form "ibm,plpks-sb-v0", indicating that the key management
+		mode is static.
+
+What:		/sys/firmware/secvar/vars/<variable name>
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	Each secure variable is represented as a directory named as
+		<variable_name>. The variable name is unique and is in ASCII
+		representation. The data and size can be determined by reading
+		their respective attribute files.
+
+		Only secvars relevant to the key management mode are exposed.
+		Only in the dynamic key management mode should the user have
+		access (read and write) to the secure boot secvars db, dbx,
+		grubdb, grubdbx, and sbat. These secvars are not consumed in the
+		static key management mode. PK, trustedcadb and moduledb are the
+		secvars common to both static and dynamic key management modes.
+
+What:		/sys/firmware/secvar/vars/<variable_name>/size
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	An integer representation of the size of the content of the
+		variable. In other words, it represents the size of the data.
+
+What:		/sys/firmware/secvar/vars/<variable_name>/data
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	A read-only file containing the value of the variable. The size
+		of the file represents the maximum size of the variable data.
+
+What:		/sys/firmware/secvar/vars/<variable_name>/update
+Date:		August 2019
+Contact:	Nayna Jain <nayna@linux.ibm.com>
+Description:	A write-only file that is used to submit the new value for the
+		variable. The size of the file represents the maximum size of
+		the variable data that can be written.