[RFC] arm64: early_ioremap fails to map ACPI MADT on 64K pages

[Yu Peng]

Hi all,

I hit an early boot failure on an arm64 system built with 64K pages while
parsing the ACPI MADT.

The failing system reports:

  PAGE_SIZE: 64K
  MADT physical address: 0x5a7ae018
  MADT length: 0x32094

The failure happens when acpi_table_parse_madt() calls into early_memremap()
via __acpi_map_table(). The MADT itself is smaller than 256K, but its
placement causes the early mapping to require 5 64K pages:

  offset within 64K page = 0x5a7ae018 & 0xffff = 0xe018
  mapped range           = PAGE_ALIGN(0xe018 + 0x32094)
                         = PAGE_ALIGN(0x400ac)
                         = 0x50000
  nrpages                = 0x50000 / 0x10000 = 5

On arm64, NR_FIX_BTMAPS is currently derived from a 256K per-slot budget:

  #define NR_FIX_BTMAPS        (SZ_256K / PAGE_SIZE)

So for 64K pages, NR_FIX_BTMAPS is 4. The mapping therefore fails the
early_ioremap() check:

  if (WARN_ON(nrpages > NR_FIX_BTMAPS))
          return NULL;

After that, MADT parsing fails and the boot continues with symptoms such as:

  ACPI: APIC not present
  missing boot CPU MPIDR, not enabling secondaries
  Kernel panic - not syncing: No interrupt controller found.

A firmware change can avoid this by placing MADT so that:

  (madt_phys & 0xffff) + madt_length <= SZ_256K

However, I do not think ACPI requires such placement, so this looks like a
kernel-side robustness issue as well, especially on large arm64 systems where
MADT can grow with CPU topology.

One possible kernel-side change is to increase the boot-time mapping budget for
CONFIG_ARM64_64K_PAGES, for example using a 512K per-slot budget only in that
configuration. I do not think this should be applied unconditionally to all
page sizes, since the arm64 early fixmap code expects the boot-ioremap range
to stay within one PMD.

Has anyone seen similar failures on arm64 64K systems?

Would maintainers prefer treating this as a firmware layout issue, or would
increasing the early_ioremap budget for 64K pages be acceptable?

Thanks,
Yu Peng