KVM: arm64: Build MPIDR to vcpu index cache at runtime

KVM: arm64: Build MPIDR to vcpu index cache at runtime

The MPIDR_EL1 register contains a unique value that identifies
the CPU. The only problem with it is that it is stupidly large
(32 bits, once the useless stuff is removed).

Trying to obtain a vcpu from an MPIDR value is a fairly common,
yet costly operation: we iterate over all the vcpus until we
find the correct one. While this is cheap for small VMs, it is
pretty expensive on large ones, specially if you are trying to
get to the one that's at the end of the list...

In order to help with this, it is important to realise that
the MPIDR values are actually structured, and that implementations
tend to use a small number of significant bits in the 32bit space.

We can use this fact to our advantage by computing a small hash
table that uses the "compression" of the significant MPIDR bits
as an index, giving us the vcpu index as a result.

Given that the MPIDR values can be supplied by userspace, and
that an evil VMM could decide to make *all* bits significant,
resulting in a 4G-entry table, we only use this method if the
resulting table fits in a single page. Otherwise, we fallback
to the good old iterative method.

Nothing uses that table just yet, but keep your eyes peeled.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Tested-by: Joey Gouly <joey.gouly@arm.com>
Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230927090911.3355209-9-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>