kexec: introduce is_kho_boot()

Patch series "efi: Fix EFI boot with kexec handover (KHO)", v3.

This patch series fixes a kernel panic that occurs when booting with both
EFI and KHO (Kexec HandOver) enabled.

The issue arises because EFI's `reserve_regions()` clears all memory
regions with `memblock_remove(0, PHYS_ADDR_MAX)` before rebuilding them
from EFI data.  This destroys KHO scratch regions that were set up early
during device tree scanning, causing a panic as the kernel has no valid
memory regions for early allocations.

The first patch introduces `is_kho_boot()` to allow early boot components
to reliably detect if the kernel was booted via KHO-enabled kexec.  The
existing `kho_is_enabled()` only checks the command line and doesn't
verify if an actual KHO FDT was passed.

The second patch modifies EFI's `reserve_regions()` to selectively remove
only non-KHO memory regions when KHO is active, preserving the critical
scratch regions while still allowing EFI to rebuild its memory map.


This patch (of 3):

During early initialisation, after a kexec, other components, like EFI
need to know if a KHO enabled kexec is performed.  The `kho_is_enabled`
function is not enough as in the early stages, it only reflects whether
the cmdline has KHO enabled, not if an actual KHO FDT exists.

Extend the KHO API with `is_kho_boot()` to provide a way for components to
check if a KHO enabled kexec is performed.

Link: https://lkml.kernel.org/r/cover.1755721529.git.epetron@amazon.de
Link: https://lkml.kernel.org/r/7dc6674a76bf6e68cca0222ccff32427699cc02e.1755721529.git.epetron@amazon.de
Signed-off-by: Evangelos Petrongonas <epetron@amazon.de>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Changyuan Lyu <changyuanl@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: vpetrog

include/linux/kexec_handover.h

@@ -40,6 +40,7 @@ struct kho_serialization;
 
 #ifdef CONFIG_KEXEC_HANDOVER
 bool kho_is_enabled(void);
+bool is_kho_boot(void);
 
 int kho_preserve_folio(struct folio *folio);
 int kho_preserve_phys(phys_addr_t phys, size_t size);
@@ -60,6 +61,11 @@ static inline bool kho_is_enabled(void)
 	return false;
 }
 
+static inline bool is_kho_boot(void)
+{
+	return false;
+}
+
 static inline int kho_preserve_folio(struct folio *folio)
 {
 	return -EOPNOTSUPP;

kernel/kexec_handover.c

@@ -951,6 +951,26 @@ static const void *kho_get_fdt(void)
 	return kho_in.fdt_phys ? phys_to_virt(kho_in.fdt_phys) : NULL;
 }
 
+/**
+ * is_kho_boot - check if current kernel was booted via KHO-enabled
+ * kexec
+ *
+ * This function checks if the current kernel was loaded through a kexec
+ * operation with KHO enabled, by verifying that a valid KHO FDT
+ * was passed.
+ *
+ * Note: This function returns reliable results only after
+ * kho_populate() has been called during early boot. Before that,
+ * it may return false even if KHO data is present.
+ *
+ * Return: true if booted via KHO-enabled kexec, false otherwise
+ */
+bool is_kho_boot(void)
+{
+	return !!kho_get_fdt();
+}
+EXPORT_SYMBOL_GPL(is_kho_boot);
+
 /**
  * kho_retrieve_subtree - retrieve a preserved sub FDT by its name.
  * @name: the name of the sub FDT passed to kho_add_subtree().