arm64: mpam: Context switch the MPAM registers

MPAM allows traffic in the SoC to be labeled by the OS, these labels are
used to apply policy in caches and bandwidth regulators, and to monitor
traffic in the SoC. The label is made up of a PARTID and PMG value. The x86
equivalent calls these CLOSID and RMID, but they don't map precisely.

MPAM has two CPU system registers that is used to hold the PARTID and PMG
values that traffic generated at each exception level will use. These can
be set per-task by the resctrl file system. (resctrl is the defacto
interface for controlling this stuff).

Add a helper to switch this.

struct task_struct's separate CLOSID and RMID fields are insufficient to
implement resctrl using MPAM, as resctrl can change the PARTID (CLOSID) and
PMG (sort of like the RMID) separately. On x86, the rmid is an independent
number, so a race that writes a mismatched closid and rmid into hardware is
benign. On arm64, the pmg bits extend the partid.
(i.e. partid-5 has a pmg-0 that is not the same as partid-6's pmg-0). In
this case, mismatching the values will 'dirty' a pmg value that resctrl
believes is clean, and is not tracking with its 'limbo' code.

To avoid this, the partid and pmg are always read and written as a
pair. This requires a new u64 field. In struct task_struct there are two
u32, rmid and closid for the x86 case, but as we can't use them here do
something else. Add this new field, mpam_partid_pmg, to struct thread_info
to avoid adding more architecture specific code to struct task_struct.
Always use READ_ONCE()/WRITE_ONCE() when accessing this field.

Resctrl allows a per-cpu 'default' value to be set, this overrides the
values when scheduling a task in the default control-group, which has
PARTID 0. The way 'code data prioritisation' gets emulated means the
register value for the default group needs to be a variable.

The current system register value is kept in a per-cpu variable to avoid
writing to the system register if the value isn't going to change. Writes
to this register may reset the hardware state for regulating bandwidth.

Finally, there is no reason to context switch these registers unless there
is a driver changing the values in struct task_struct. Hide the whole thing
behind a static key. This also allows the driver to disable MPAM in
response to errors reported by hardware. Move the existing static key to
belong to the arch code, as in the future the MPAM driver may become a
loadable module.

All this should depend on whether there is an MPAM driver, hide it behind
CONFIG_ARM64_MPAM.

Tested-by: Gavin Shan <gshan@redhat.com>
Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Tested-by: Peter Newman <peternewman@google.com>
Tested-by: Zeng Heng <zengheng4@huawei.com>
Tested-by: Punit Agrawal <punit.agrawal@oss.qualcomm.com>
Tested-by: Jesse Chick <jessechick@os.amperecomputing.com>
CC: Amit Singh Tomar <amitsinght@marvell.com>
Reviewed-by: Zeng Heng <zengheng4@huawei.com>
Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Co-developed-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>