arm64-64k: Increase max NR_IRQS from 64+8192 to 2^^19

For large HPC users of arm64 that will be using the
64k page-sized kernel variant, increase the needed max
number of irqs from the current default, which is 64+8192,
to a desired 2^^19.

This is a RHEL-only patch.
It is based on the original upstream posting, which
was rejected because it put too large a memory increase
for small memory arm64 systems.
In this RHEL-only variation, we only change it for the
64k page-size variant, arm64 kernel b/c that's the use-case,
and doesn't impact the 4k page-sized (standard) arm64 kernel.
Note: A RHEL8 kernel on a laptop has 524544(512K+256) as NR_IRQs,
      so 2^^19/512K is not unreasonable for a server,
      but arm64-4k may be in smaller, edge servers.

The increase in memory for the fixed-size array that
NR_IRQS sizes is approx. 256K bytes, which is deemed
small for arm64-64k memory configurations.

The accepted upstream patch for this increase was based on
a dynamic, maple-tree design.  It is too late in the
release to pull in the core maple-tree support, and base
this functionality with little use seen upstream.
Note: An attempt to base it on XArray, which is in RHEL9,
      failed upstream, thus the (successful) maple-tree
      patch accepted upstream.

This patch will be reverted and replaced with the upstream
version, based on maple-tree, in the next release, when
the core maple-tree support will be backported (in kernel
and in crash for use in crash-analysis).

Upstream Status: RHEL only
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2166450

Testing:
Simple boot testing and check KERN_INFO output of NR_IRQS.

v2: set Upstream status to RHEL only.
v3: rebase to tip, removing MR1820 dependency.

Signed-off-by: Donald Dutile <ddutile@redhat.com>

Author: ddutile

arch/arm64/include/asm/irq.h

@@ -4,6 +4,15 @@
 
 #ifndef __ASSEMBLER__
 
+
+/*
+ * RHEL-9.2-only -- increase max NR_IRQS from 64+8192 default
+ *    for large, HPC arm64 kernels with 64K page-size
+ */
+#if defined(CONFIG_ARM_GIC_V3_ITS) && defined(CONFIG_ARM64_64K_PAGES)
+#define  NR_IRQS  (1 << 19)
+#endif
+
 #include <asm-generic/irq.h>
 
 struct pt_regs;