Re: [RFC PATCH 2/3] powerpc: add support for Kexec HandOver (KHO)
From: Sourabh Jain
Date: Sun Aug 23 2026 - 11:41:46 EST
On 21/08/26 17:26, Pratyush Yadav wrote:
On Fri, Aug 21 2026, Sourabh Jain wrote:
Add the architecture bits needed to enable CONFIG_KEXEC_HANDOVER oncrashkernel has the variant "crashkernel=size[KMG],high", which ensures
powerpc.
Set ARCH_SUPPORTS_KEXEC_HANDOVER for PPC64, following the existing
pattern used by ARCH_SUPPORTS_KEXEC and ARCH_SUPPORTS_KEXEC_FILE.
On the boot path, parse the "linux,kho-fdt" and "linux,kho-scratch"
properties from /chosen and pass them to kho_populate(). This lets a
kernel booted via KHO kexec recover the FDT and scratch region left
behind by the previous kernel. The call is placed early in
setup_arch(), before unflatten_device_tree().
Open issues:
============
This patch also adds "depends on !CRASH_DUMP" to
ARCH_SUPPORTS_KEXEC_HANDOVER. This is needed because of an ordering
conflict between crashkernel reservation and KHO scratch reservation
on powerpc.
Crashkernel memory is reserved very early in boot, from arch-specific
code: head.S -> early_setup() -> early_init_devtree() ->
arch_reserve_crashkernel() / fadump_reserve_mem(). KHO's scratch
region is reserved later, from generic code: start_kernel() ->
mm_core_init() -> kho_memory_init(). So on powerpc, crashkernel
memory is always reserved first.
This ordering causes a real failure. In the common case, crashkernel
reservation on powerpc starts at a 512M offset (the exact offset can
vary, but 512M is typical). So with crashkernel=3G, the reservation
occupies memory from 512M up to 3.5G -- roughly 75% of the entire low
4G area.
Since crashkernel reservation always happens first, that 3G is
already committed by the time kho_memory_init() runs. It then tries
to reserve a low scratch region sized at 200% of whatever is already
reserved below 4G. With ~75% of that 4G area already taken by
crashkernel memory, 200% of that easily exceeds the remaining space
-- and since the low scratch region is itself capped at 4G, there's
no room left to fit it. The reservation fails.
memory is allocated above 4G. Unless powerpc has some requirement for
strictly having the crashkernel below 4G, I think it will make a lot of
sense to enable support for this feature. So KHO users can specify this
to get crashkernel working with KHO.
I agree that this is one way to work around the low-memory reservation problem.
However, there are a few things that come into play here:
1. On powerpc, the crashkernel reservation can go up to 64 GB for kdump. With the
current default scratch memory reservation policy, this could result in reserving
up to 256 GB of scratch memory: 200% for the high-memory reservation and another
200% for per-node memory.
For fadump, which is the powerpc-specific memory dump capture mechanism, the crashkernel
reservation can go up to 180 GB. In this case, we could end up reserving up to 720 GB of
scratch memory, which is too much. I agree that users can tune this, but I think the
default scale should be more reasonable for powerpc.
2. Fadump also uses the crashkernel kernel command-line argument, but its reservation policy
is different from kdump. The crashkernel base address starts after the memory needed for
fadump. For example, with crashkernel=3G, the base address would be 3 GB, and the crashkernel
reservation would be from 3 GB to 6 GB. So, depending on the crashkernel size, the reservation
may or may not fall within low memory. Also, fadump does not support crashkernel=xxM,high.
3. I do have a patch [1] to support high crashkernel reservations with kdump, but this would not
work with Hash MMU. With Hash MMU, the kernel image is constrained to low memory, whereas with
crashkernel=,high, all segments would be loaded into high memory.
So, while supporting crashkernel=,high can help address the low-memory reservation issue for kdump,
I think there are still some powerpc-specific constraints to consider. I would like to explore
whether we can fix the ordering between crashkernel and scratch memory reservations, so that we can
avoid unnecessarily large scratch memory reservations and address some of the other constraints mentioned
above. Please share your thoughts.
Powerpc doesn't support this right now, but from a quick skim of the
code, I think it should be simple enough.
I have patch series under review for the same:
[1] https://lore.kernel.org/all/20260708143357.673251-1-sourabhjain@xxxxxxxxxxxxx/
From
arch_reserve_crashkernel() you just need to pass a bool * to
parse_crashkernel(), and then pass the result to
reserve_crashkernel_generic().
Due to some architecture-specific dependencies (such as RTAS), booting the kernel from above 4G with
support for high crashkernel is not as straightforward as on other architectures. Patch 2/4 in [1] has the
details.
Solving the ordering of crash reservations and KHO is tricky and comes
with some difficult tradeoffs. Allocating crash from highmem should be a
lot simpler.
Could you please elaborate a bit on what makes the ordering tricky and what the
main tradeoffs are between crash reservations and KHO? It would help me better
understand the concerns here.
And on that note, I don't think you should do a depends on !CRASH_DUMP.
Even when CONFIG_KEXEC_HANDOVER is enabled, KHO isn't on by default
(well, unless KEXEC_HANDOVER_ENABLE_DEFAULT is set). You need to enable
it via cmdline. So it is entirely possible for people using KHO on PPC
to not use crash and vice versa. This decision can be made at deployment
time, not at compile time.
Agree. depends on !CRASH_DUMP is temporary and will be removed once
we settle the crashkernel reservation and scratch region handling.
To work around this and get KHO working on powerpc, this patch:[...]
1. Makes KHO usable on powerpc only when CRASH_DUMP is disabled.
2. Calls kho_populate() from setup_arch(), so it runs before
kho_memory_init() reserves the scratch region.
The real fix would be to reserve the KHO scratch region before
crashkernel memory instead of after. But scratch reservation happens
in generic code (kho_memory_init(), called from mm_core_init()), so
this isn't something powerpc can address on its own -- it needs
discussion on how to influence the ordering between generic scratch
reservation and arch-specific crashkernel reservation. This patch
doesn't attempt that; it's meant as a starting point for that
discussion.
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig[...]
index 2580e27e4328..61350d3e7a19 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -716,6 +716,11 @@ config ARCH_SELECTS_CRASH_DUMP
depends on CRASH_DUMP
select RELOCATABLE if PPC64 || 44x || PPC_85xx
+config ARCH_SUPPORTS_KEXEC_HANDOVER
+ def_bool y
+ depends on PPC64
+ depends on !CRASH_DUMP
+
config ARCH_SUPPORTS_CRASH_HOTPLUG
def_bool y
depends on PPC64
diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
index 4afaba19b586..1fee743abdf2 100644
--- a/arch/powerpc/kernel/setup-common.c
+++ b/arch/powerpc/kernel/setup-common.c
}This looks pretty much a duplicate of early_init_dt_check_kho(). On
#endif
+#ifdef CONFIG_PPC64
+static void __init init_kho(const void *fdt)
+{
+ unsigned long node;
+ u64 fdt_start, fdt_size, scratch_start, scratch_size;
+
+ if (!IS_ENABLED(CONFIG_KEXEC_HANDOVER))
+ return;
+
+ /* Find and verify the /chosen node, same as early_init_dt_scan_chosen() does */
+ node = fdt_path_offset(fdt, "/chosen");
+ if ((long)node < 0)
+ node = fdt_path_offset(fdt, "/chosen@0");
+ if ((long)node < 0)
+ return;
+
+ if (!of_flat_dt_get_addr_size(node, "linux,kho-fdt",
+ &fdt_start, &fdt_size))
+ return;
+ if (!of_flat_dt_get_addr_size(node, "linux,kho-scratch",
+ &scratch_start, &scratch_size))
+ return;
+
+ kho_populate(fdt_start, fdt_size, scratch_start, scratch_size);
+}
arm64 this is called via early_init_dt_scan(). But from a quick search I
don't see powerpc calling it.
Yes it is not called on powerpc.
Would it make sense to call this function (or
early_init_dt_scan_nodes()) for powerpc?
If not, I think it would be a better idea to expose
early_init_dt_check_kho() and call it from powerpc setup_arch() instead
of duplicating the logic.
Agree they are identical.
I would prefer making early_init_dt_check_kho() as public function and call from arch specific code. Thanks for the review. - Sourabh Jain
+#endif[...]
+
/*
* Called into from start_kernel this initializes memblock, which is used
* to manage page allocation until mem_init is called.