The DIC and IDC bits in the CTR_EL0 register signal to the kernel when it
can relax the instruction cache synchronisation operations. The IDC bit
means we can relax cleaning the data cache to the point of unification
while the DIC bit means we don't need to invalidate the instruction cache
for data coherence. In both cases an appropriate barrier is still needed.
For now only implement the case where both bits are set, as is the case
on the Neoverse-N1 as used in the Amazon AWS Graviton 2 CPU. Note that
this behaviour is a optional on the N1 so we may later need to implement
only one or the other bit being set.
There is a tunable to disable each flag on boot.
Testing on a 4 core Graviton 2 instance found a significant improvement
in sys and real time when running "make buildkernel -j4", with no
significant difference in user time.
Reviewed by: markj
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D24853
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