arm64: Reimplement mmu_disable() in assembly

Disabling the MMU with proper cache behavior is a bit tricky on ARM64:
you can flush the cache first and then disable the MMU (like we have
been doing), but then you run the risk of having new cache lines
allocated in the tiny window between the two, which may or may not
become a problem when those get flushed at a later point (on some
platforms certain memory regions "go away" at certain points in a way
that makes the CPU very unhappy if it ever issues a write cycle to
them again afterwards).

The obvious alternative is to first disable the MMU and then flush the
cache, ensuring that every memory access after the flush already has the
non-cacheable attribute. But we can't just flip the order around in the
C code that we have because then those accesses in the tiny window
in-between will go straight to memory, so loads may yield the wrong
result or stores may get overwritten again by the later cache flush.

In the end, this all shouldn't really be a problem because we can do
both operations purely from registers without doing any explicit memory
accesses in-between. We just have to reimplement the function in
assembly to make sure the compiler doesn't insert any stack accesses at
the wrong points.

Change-Id: Ic552960c91400dadae6f130b2521a696eeb4c0b1
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/27238
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
3 files changed