[PATCH v5 0/3] Increase the number of IRQ descriptors for SPARSEIRQ

[Shanker Donthineni <sdonthineni@nvidia.com>]

The ARM64 architecture uses SPARSEIRQ with a default value of NR_IRQS,
which is set to 64. This means that only 64+8192 IRQ descriptors are
allowed, which may not be sufficient for modern ARM64 servers that
have a large number of IO devices and GIC hardware that supports
direct vSGI and vLPI injection features.

This limitation has caused issues when attempting to launch multiple
virtual machines with GICv4.1 features, resulting in the error message
'kvm_err("VPE IRQ allocation failure\n")'. The root cause of this issue
is the ~8K IRQ descriptor limit.

To address this issue, an initial proposal was made to define NR_IRQS
to 2^19 for ARM64. However, Marc Zyngier suggested implementing a
generic solution instead of hard-coded values. Thomas Gleixner advised
to use the maple tree data structure and provided most of the necessary
functions.

For more information, refer to the discussion thread at
https://lore.kernel.org/linux-arm-kernel/20230104023738.1258925-1-sdonthineni@nvidia.com/.

This patch series converts the static memory allocation to dynamic using
the maple tree, and increases the maximum number of IRQ descriptors to
INT_MAX from NR_IRQS+8192. This change has been tested on an ARM64 server
with CONFIG_SPARSE_IRQ=y, where 256 virtual machines were launched,
creating a total of 128K+ IRQ descriptors, and IRQ injection was verified.

Teted on ARM64 system:
  - Normal boot with ~200 cores
  - CPU hot-unplug/hot-plug using sysfs
  - Booted virtual machines with PCIe device pass-through
  - Hot-unplug of CPU where vfio-pci interrupts are bounded

Changes in v5:
  - Change function name irq_find_next_irq() to irq_find_at_or_after()
  - Update comment to reflect the return value of irq_get_next_irq()

Changes in v4:
  - Fix the iterator function using mt_find() instead of nt_next()
  - Tested CPU hot-unplug and hot-plug on ARM64 system. This is
    completely broken before v4.

Changes in v3:
  - Edited commit text
  - Added a helper function irq_resend_init()
  - Rebased to v6.3-rc6

Changes in v2:
  - The patches have been updated to v6.3-rc5.
  - The patches 2/5 and 4/5 have been removed as they are unnecessary.
  - The review comments from Thomas have been addressed.


Shanker Donthineni (3):
  genirq: Use hlist for managing resend handlers
  genirq: Encapsulate sparse bitmap handling
  genirq: Use the maple tree for IRQ descriptors management

 include/linux/irqdesc.h |  3 ++
 kernel/irq/chip.c       |  1 +
 kernel/irq/internals.h  |  6 ++--
 kernel/irq/irqdesc.c    | 77 +++++++++++++++++++++++++----------------
 kernel/irq/resend.c     | 47 ++++++++++++++++---------
 5 files changed, 86 insertions(+), 48 deletions(-)

-- 
2.25.1