[RFC PATCH 0/3] powerpc: initial support for Kexec HandOver (KHO)

[RFC PATCH 0/3] powerpc: initial support for Kexec HandOver (KHO)

[Sourabh Jain]

This series adds initial powerpc support for Kexec HandOver (KHO),
allowing state to be preserved across kexec on powerpc. Posting as RFC
because some of the patches are workarounds for issues I ran into, and
I'd like input on how best to address them.

Patch 1 is a small Kconfig fix needed to unblock patch 2. It won't be
needed once the ordering problem described under patch 2 is
addressed properly.

Patch 2 wires up ARCH_SUPPORTS_KEXEC_HANDOVER and adds the code to
retrieve the KHO FDT and scratch region from /chosen on boot. On
powerpc, crashkernel memory is reserved very early, from arch-specific
code, while KHO's scratch region is reserved later, from generic code
(kho_memory_init()). Crashkernel reservation always runs first, so by
the time KHO reserves its scratch region, not enough memory is left
and the reservation fails. Until scratch reservation can be moved
ahead of crashkernel reservation, this patch makes KHO and CRASH_DUMP
mutually exclusive on powerpc as a stopgap. The failure scenario and
the reasoning behind this restriction are described in the commit
message with an example.

Patch 3 excludes TRANSPARENT_HUGEPAGE and HUGETLB_PAGE from
ARCH_SUPPORTS_KEXEC_HANDOVER, since KHO's static_assert() on
SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES fails to build
when either is enabled -- pageblock_order isn't a compile-time
constant on powerpc in that configuration.

With all three patches applied, CONFIG_TEST_KEXEC_HANDOVER passes on
powerpc.

Open questions I'd appreciate feedback on:

 - How should KHO scratch reservation and powerpc's crashkernel
   reservation be ordered so both can be enabled together, given that
   scratch reservation currently happens in generic code
   (kho_memory_init())?

- Excluding THP/HUGETLB_PAGE is a workaround for the build failure.
  Looking for input on how to handle CMA_MIN_ALIGNMENT_BYTES when
  pageblock_order is non-constant.

Cc: Aditya Gupta <adityag@linux.ibm.com>
Cc: Alexander Graf <graf@amazon.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Pratyush Yadav <pratyush@kernel.org>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Sourabh Jain <sourabhjain@linux.ibm.com>
Cc: kexec@lists.infradead.org
Cc: linux-kernel@vger.kernel.org

Sourabh Jain (3):
  kernel/liveupdate: make KEXEC_HANDOVER depend on KEXEC_FILE instead of
    selecting it
  powerpc: add support for Kexec HandOver (KHO)
  powerpc/kho: exclude THP and HUGETLB_PAGE

 arch/powerpc/Kconfig               |  6 ++++++
 arch/powerpc/kernel/setup-common.c | 33 ++++++++++++++++++++++++++++++
 kernel/liveupdate/Kconfig          |  2 +-
 3 files changed, 40 insertions(+), 1 deletion(-)

-- 
2.55.0

[RFC PATCH 1/3] kernel/liveupdate: make KEXEC_HANDOVER depend on KEXEC_FILE instead of selecting it

[Sourabh Jain]

An upcoming patch makes ARCH_SUPPORTS_KEXEC_HANDOVER depend on
CRASH_DUMP on powerpc:

    config ARCH_SUPPORTS_KEXEC_HANDOVER
    	def_bool y
    	depends on PPC64
    	depends on !CRASH_DUMP

That triggers a recursive dependency error:

    error: recursive dependency detected!
    	symbol KEXEC_HANDOVER depends on ARCH_SUPPORTS_KEXEC_HANDOVER
    	symbol ARCH_SUPPORTS_KEXEC_HANDOVER depends on CRASH_DUMP
    	symbol CRASH_DUMP depends on KEXEC_CORE
    	symbol KEXEC_CORE is selected by KEXEC_FILE
    	symbol KEXEC_FILE is selected by KEXEC_HANDOVER

KEXEC_HANDOVER and KEXEC_CORE end up depending on each other when
ARCH_SUPPORTS_KEXEC_HANDOVER depends on CRASH_DUMP. This creates a
recursive Kconfig dependency.

Change KEXEC_FILE from select to depends on to break this cycle.
This means KEXEC_FILE must be enabled explicitly instead of being
enabled automatically by KEXEC_HANDOVER.

Keep this change until the powerpc-specific KHO and CRASH_DUMP
dependency is resolved.

Cc: Aditya Gupta <adityag@linux.ibm.com>
Cc: Alexander Graf <graf@amazon.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Pratyush Yadav <pratyush@kernel.org>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: kexec@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
---
 kernel/liveupdate/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/liveupdate/Kconfig b/kernel/liveupdate/Kconfig
index c13af38ba23a..22e317dc1e1a 100644
--- a/kernel/liveupdate/Kconfig
+++ b/kernel/liveupdate/Kconfig
@@ -6,7 +6,7 @@ config KEXEC_HANDOVER
 	bool "kexec handover"
 	depends on ARCH_SUPPORTS_KEXEC_HANDOVER && ARCH_SUPPORTS_KEXEC_FILE
 	select MEMBLOCK_KHO_SCRATCH
-	select KEXEC_FILE
+	depends on KEXEC_FILE
 	select LIBFDT
 	select CMA
 	help
-- 
2.55.0

[RFC PATCH 2/3] powerpc: add support for Kexec HandOver (KHO)

[Sourabh Jain]

Add the architecture bits needed to enable CONFIG_KEXEC_HANDOVER on
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.

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.

Together with patch 3, this gets CONFIG_TEST_KEXEC_HANDOVER passing
on powerpc.

Cc: Aditya Gupta <adityag@linux.ibm.com>
Cc: Alexander Graf <graf@amazon.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Pratyush Yadav <pratyush@kernel.org>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: kexec@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
---
 arch/powerpc/Kconfig               |  5 +++++
 arch/powerpc/kernel/setup-common.c | 33 ++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

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
@@ -35,6 +35,8 @@
 #include <linux/of_irq.h>
 #include <linux/hugetlb.h>
 #include <linux/pgtable.h>
+#include <linux/libfdt.h>
+#include <linux/kexec_handover.h>
 #include <asm/io.h>
 #include <asm/paca.h>
 #include <asm/processor.h>
@@ -910,6 +912,33 @@ static void __init smp_setup_pacas(void)
 }
 #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);
+}
+#endif
+
 /*
  * Called into from start_kernel this initializes memblock, which is used
  * to manage page allocation until mem_init is called.
@@ -923,6 +952,10 @@ void __init setup_arch(char **cmdline_p)
 	/* Set a half-reasonable default so udelay does something sensible */
 	loops_per_jiffy = 500000000 / HZ;
 
+#ifdef CONFIG_PPC64
+	init_kho(initial_boot_params);
+#endif
+
 	/* Unflatten the device-tree passed by prom_init or kexec */
 	unflatten_device_tree();
 
-- 
2.55.0

Re: [RFC PATCH 2/3] powerpc: add support for Kexec HandOver (KHO)

[Pratyush Yadav]

On Fri, Aug 21 2026, Sourabh Jain wrote:

> Add the architecture bits needed to enable CONFIG_KEXEC_HANDOVER on
> 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.

crashkernel has the variant "crashkernel=size[KMG],high", which ensures
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.

Powerpc doesn't support this right now, but from a quick skim of the
code, I think it should be simple enough. From
arch_reserve_crashkernel() you just need to pass a bool * to
parse_crashkernel(), and then pass the result to
reserve_crashkernel_generic().

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.

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.

>
> 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
[...]
>  }
>  #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);
> +}

This looks pretty much a duplicate of early_init_dt_check_kho(). On
arm64 this is called via early_init_dt_scan(). But from a quick search I
don't see powerpc calling it.

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.

> +#endif
> +
>  /*
>   * Called into from start_kernel this initializes memblock, which is used
>   * to manage page allocation until mem_init is called.
[...]

-- 
Regards,
Pratyush Yadav

Re: [RFC PATCH 2/3] powerpc: add support for Kexec HandOver (KHO)

[Sourabh Jain]

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 on
>> 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.
> crashkernel has the variant "crashkernel=size[KMG],high", which ensures
> 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@linux.ibm.com/

> 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
> [...]
>>   }
>>   #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);
>> +}
> This looks pretty much a duplicate of early_init_dt_check_kho(). On
> 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.
> [...]
>

[RFC PATCH 3/3] powerpc/kho: exclude THP and HUGETLB_PAGE

[Sourabh Jain]

Enabling CONFIG_KEXEC_HANDOVER together with CONFIG_TRANSPARENT_HUGEPAGE
or CONFIG_HUGETLB_PAGE on powerpc fails to build:

    kernel/liveupdate/kexec_handover.c:49:39: error: expression in static assertion is not constant
       49 | static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);
    ./include/linux/build_bug.h:80:56: note: in definition of macro '__static_assert'
       80 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
          |                                                        ^~~~
    kernel/liveupdate/kexec_handover.c:49:1: note: in expansion of macro 'static_assert'
       49 | static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);

CMA_MIN_ALIGNMENT_BYTES is derived from pageblock_order, which on
powerpc is not a compile-time constant when THP or HUGETLB_PAGE is
enabled -- powerpc can determine huge page size at runtime in that
configuration, so pageblock_order becomes a variable rather than a
constant expression. KHO's static_assert() on
SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES then fails to
compile, since it requires both sides to be constant expressions.

Until CMA_MIN_ALIGNMENT_BYTES (or KHO's use of it) is made to handle
the non-constant pageblock_order case on powerpc, exclude
TRANSPARENT_HUGEPAGE and HUGETLB_PAGE from ARCH_SUPPORTS_KEXEC_HANDOVER
so the two can't be enabled together and hit this build failure.

Cc: Aditya Gupta <adityag@linux.ibm.com>
Cc: Alexander Graf <graf@amazon.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Pratyush Yadav <pratyush@kernel.org>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: kexec@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
---
 arch/powerpc/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 61350d3e7a19..f68b9fc883ba 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -720,6 +720,7 @@ config ARCH_SUPPORTS_KEXEC_HANDOVER
 	def_bool y
 	depends on PPC64
 	depends on !CRASH_DUMP
+	depends on !TRANSPARENT_HUGEPAGE && !HUGETLB_PAGE
 
 config ARCH_SUPPORTS_CRASH_HOTPLUG
 	def_bool y
-- 
2.55.0

Re: [RFC PATCH 3/3] powerpc/kho: exclude THP and HUGETLB_PAGE

[Pratyush Yadav]

On Fri, Aug 21 2026, Sourabh Jain wrote:

> Enabling CONFIG_KEXEC_HANDOVER together with CONFIG_TRANSPARENT_HUGEPAGE
> or CONFIG_HUGETLB_PAGE on powerpc fails to build:
>
>     kernel/liveupdate/kexec_handover.c:49:39: error: expression in static assertion is not constant
>        49 | static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);
>     ./include/linux/build_bug.h:80:56: note: in definition of macro '__static_assert'
>        80 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
>           |                                                        ^~~~
>     kernel/liveupdate/kexec_handover.c:49:1: note: in expansion of macro 'static_assert'
>        49 | static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);
>
> CMA_MIN_ALIGNMENT_BYTES is derived from pageblock_order, which on
> powerpc is not a compile-time constant when THP or HUGETLB_PAGE is
> enabled -- powerpc can determine huge page size at runtime in that
> configuration, so pageblock_order becomes a variable rather than a
> constant expression. KHO's static_assert() on
> SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES then fails to
> compile, since it requires both sides to be constant expressions.
>
> Until CMA_MIN_ALIGNMENT_BYTES (or KHO's use of it) is made to handle
> the non-constant pageblock_order case on powerpc, exclude
> TRANSPARENT_HUGEPAGE and HUGETLB_PAGE from ARCH_SUPPORTS_KEXEC_HANDOVER
> so the two can't be enabled together and hit this build failure.

This should be a part of patch 2 I think. It is not a good idea to break
the build in one patch and fix it in the next.

That said, this is a debug check of sorts since SCRATCH_ALIGNMENT_BYTES
is of MAX_ORDER_NR_PAGES, and pageblocks _should_ always be smaller of
equal to this.

Anyway, I don't think this is something you should gate by a config. I
think you should either remove the static_assert(), or turn it into a
runtime check.

+Cc Michal.

>
> Cc: Aditya Gupta <adityag@linux.ibm.com>
> Cc: Alexander Graf <graf@amazon.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Baoquan He <baoquan.he@linux.dev>
> Cc: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
> Cc: Hari Bathini <hbathini@linux.ibm.com>
> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
> Cc: Pratyush Yadav <pratyush@kernel.org>
> Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
> Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
> Cc: kexec@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> ---
>  arch/powerpc/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 61350d3e7a19..f68b9fc883ba 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -720,6 +720,7 @@ config ARCH_SUPPORTS_KEXEC_HANDOVER
>  	def_bool y
>  	depends on PPC64
>  	depends on !CRASH_DUMP
> +	depends on !TRANSPARENT_HUGEPAGE && !HUGETLB_PAGE
>  
>  config ARCH_SUPPORTS_CRASH_HOTPLUG
>  	def_bool y

-- 
Regards,
Pratyush Yadav

Re: [RFC PATCH 3/3] powerpc/kho: exclude THP and HUGETLB_PAGE

[Sourabh Jain]

On 21/08/26 17:04, Pratyush Yadav wrote:
> On Fri, Aug 21 2026, Sourabh Jain wrote:
>
>> Enabling CONFIG_KEXEC_HANDOVER together with CONFIG_TRANSPARENT_HUGEPAGE
>> or CONFIG_HUGETLB_PAGE on powerpc fails to build:
>>
>>      kernel/liveupdate/kexec_handover.c:49:39: error: expression in static assertion is not constant
>>         49 | static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);
>>      ./include/linux/build_bug.h:80:56: note: in definition of macro '__static_assert'
>>         80 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
>>            |                                                        ^~~~
>>      kernel/liveupdate/kexec_handover.c:49:1: note: in expansion of macro 'static_assert'
>>         49 | static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);
>>
>> CMA_MIN_ALIGNMENT_BYTES is derived from pageblock_order, which on
>> powerpc is not a compile-time constant when THP or HUGETLB_PAGE is
>> enabled -- powerpc can determine huge page size at runtime in that
>> configuration, so pageblock_order becomes a variable rather than a
>> constant expression. KHO's static_assert() on
>> SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES then fails to
>> compile, since it requires both sides to be constant expressions.
>>
>> Until CMA_MIN_ALIGNMENT_BYTES (or KHO's use of it) is made to handle
>> the non-constant pageblock_order case on powerpc, exclude
>> TRANSPARENT_HUGEPAGE and HUGETLB_PAGE from ARCH_SUPPORTS_KEXEC_HANDOVER
>> so the two can't be enabled together and hit this build failure.
> This should be a part of patch 2 I think. It is not a good idea to break
> the build in one patch and fix it in the next.

Agree. Since this is an RFC patch series, I kept it separate so that 
this issue could
be reviewed independently from patch 2. Going forward, I will merge them.


>
> That said, this is a debug check of sorts since SCRATCH_ALIGNMENT_BYTES
> is of MAX_ORDER_NR_PAGES, and pageblocks _should_ always be smaller of
> equal to this.
>
> Anyway, I don't think this is something you should gate by a config. I
> think you should either remove the static_assert(), or turn it into a
> runtime check.

Agreed. I will evaluate both options and decide which approach makes 
more sense.

Thanks for the review Pratyush.

- Sourabh Jain


>
> +Cc Michal.
>
>> Cc: Aditya Gupta <adityag@linux.ibm.com>
>> Cc: Alexander Graf <graf@amazon.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Baoquan He <baoquan.he@linux.dev>
>> Cc: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
>> Cc: Hari Bathini <hbathini@linux.ibm.com>
>> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
>> Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Mike Rapoport <rppt@kernel.org>
>> Cc: Nicholas Piggin <npiggin@gmail.com>
>> Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
>> Cc: Pratyush Yadav <pratyush@kernel.org>
>> Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>> Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
>> Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
>> Cc: kexec@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
>> ---
>>   arch/powerpc/Kconfig | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>> index 61350d3e7a19..f68b9fc883ba 100644
>> --- a/arch/powerpc/Kconfig
>> +++ b/arch/powerpc/Kconfig
>> @@ -720,6 +720,7 @@ config ARCH_SUPPORTS_KEXEC_HANDOVER
>>   	def_bool y
>>   	depends on PPC64
>>   	depends on !CRASH_DUMP
>> +	depends on !TRANSPARENT_HUGEPAGE && !HUGETLB_PAGE
>>   
>>   config ARCH_SUPPORTS_CRASH_HOTPLUG
>>   	def_bool y