tree a15f01e8abe216186a37dc18e1ed904a87abc6fa
parent 1cb1ed2c4be43d7fe6c29886b4d0037dced3ae3c
author Boyan Karatotev <boyan.karatotev@arm.com> 1732529666 +0000
committer Boyan Karatotev <boyan.karatotev@arm.com> 1740473445 +0000

perf(mpmm): greatly simplify MPMM enablement

MPMM is a core-specific microarchitectural feature. It has been present
in every Arm core since the Cortex-A510 and has been implemented in
exactly the same way. Despite that, it is enabled more like an
architectural feature with a top level enable flag. This utilised the
identical implementation.

This duality has left MPMM in an awkward place, where its enablement
should be generic, like an architectural feature, but since it is not,
it should also be core-specific if it ever changes. One choice to do
this has been through the device tree.

This has worked just fine so far, however, recent implementations expose
a weakness in that this is rather slow - the device tree has to be read,
there's a long call stack of functions with many branches, and system
registers are read. In the hot path of PSCI CPU powerdown, this has a
significant and measurable impact. Besides it being a rather large
amount of code that is difficult to understand.

Since MPMM is a microarchitectural feature, its correct placement is in
the reset function. The essence of the current enablement is to write
CPUPPMCR_EL3.MPMM_EN if CPUPPMCR_EL3.MPMMPINCTL == 0. Replacing the C
enablement with an assembly macro in each CPU's reset function achieves
the same effect with just a single close branch and a grand total of 6
instructions (versus the old 2 branches and 32 instructions).

Having done this, the device tree entry becomes redundant. Should a core
that doesn't support MPMM arise, this can cleanly be handled in the
reset function. As such, the whole ENABLE_MPMM_FCONF and platform hooks
mechanisms become obsolete and are removed.

Change-Id: I1d0475b21a1625bb3519f513ba109284f973ffdf
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>