Hi Marc,

On 29/04/2021 14:35, Marc Zyngier wrote:

It recently became apparent that using kexec with kexec_file_load() on
arm64 is pretty similar to playing Russian roulette.

Depending on the amount of memory, the HW supported and the firmware
interface used, your secondary kernel may overwrite critical memory
regions without which the secondary kernel cannot boot (the GICv3 LPI
tables being a prime example of such reserved regions).

It turns out that there is at least two ways for reserved memory
regions to be described to kexec: /proc/iomem for the userspace
implementation, and memblock.reserved for kexec_file. 

One is spilled into the other by request_standard_resources()...

And of course,
our LPI tables are only reserved using the resource tree, leading to
the aforementioned stamping.

Presumably well after efi_init() has run...

Similar things could happen with ACPI tables as well.

efi_init() calls reserve_regions(), which has:
|	/* keep ACPI reclaim memory intact for kexec etc. */
|	if (md->type == EFI_ACPI_RECLAIM_MEMORY)
|		memblock_reserve(paddr, size);

This is also what stops mm from allocating them, as memblock-reserved gets copied into the
PG_Reserved flag by free_low_memory_core_early()'s calls to reserve_bootmem_region().

Is your machines firmware putting them in a region with a different type?

(The UEFI spec has something to say: see 2.3.6 "AArch64 Platforms":
| ACPI Tables loaded at boot time can be contained in memory of type EfiACPIReclaimMemory
| (recommended) or EfiACPIMemoryNVS

NVS would fail the is_usable_memory() check earlier, so gets treated as nomap)


Thanks,

James

On my 24xA53 system artificially limited to 256MB of RAM (yes, it
boots with that little memory), trying to kexec a secondary kernel
failed every times. I can only presume that this was mostly tested
using kdump, which preserves the entire kernel memory range.

This small series aims at triggering a discussion on what are the
expectations for kexec_file, and whether we should unify the two
reservation mechanisms.

And in the meantime, it gets things going...

Marc Zyngier (2):
  firmware/efi: Tell memblock about EFI reservations
  ACPI: arm64: Reserve the ACPI tables in memblock

 arch/arm64/kernel/acpi.c   |  1 +
 drivers/firmware/efi/efi.c | 23 ++++++++++++++++++++++-
 2 files changed, 23 insertions(+), 1 deletion(-)

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