kho: introduce high-level memory allocation API

Currently, clients of KHO must manually allocate memory (e.g., via
alloc_pages), calculate the page order, and explicitly call
kho_preserve_folio().  Similarly, cleanup requires separate calls to
unpreserve and free the memory.

Introduce a high-level API to streamline this common pattern:

- kho_alloc_preserve(size): Allocates physically contiguous, zeroed
  memory and immediately marks it for preservation.
- kho_unpreserve_free(ptr): Unpreserves and frees the memory
  in the current kernel.
- kho_restore_free(ptr): Restores the struct page state of
  preserved memory in the new kernel and immediately frees it to the
  page allocator.

[pasha.tatashin@soleen.com: build fixes]
  Link: https://lkml.kernel.org/r/CA+CK2bBgXDhrHwTVgxrw7YTQ-0=LgW0t66CwPCgG=C85ftz4zw@mail.gmail.com
Link: https://lkml.kernel.org/r/20251114190002.3311679-4-pasha.tatashin@soleen.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Baoquan He <bhe@redhat.com>
Cc: Coiby Xu <coxu@redhat.com>
Cc: Dave Vasilevsky <dave@vasilevsky.ca>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Kees Cook <kees@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 15 insertions, 7 deletions

diff --git a/include/linux/kexec_handover.h b/include/linux/kexec_handover.h
index 80ece4232617..dde952227b88 100644
--- a/include/linux/kexec_handover.h
+++ b/include/linux/kexec_handover.h
@@ -2,8 +2,9 @@
 #ifndef LINUX_KEXEC_HANDOVER_H
 #define LINUX_KEXEC_HANDOVER_H
 
-#include <linux/types.h>
+#include <linux/err.h>
 #include <linux/errno.h>
+#include <linux/types.h>
 
 struct kho_scratch {
 	phys_addr_t addr;
@@ -48,6 +49,9 @@ int kho_preserve_pages(struct page *page, unsigned int nr_pages);
 int kho_unpreserve_pages(struct page *page, unsigned int nr_pages);
 int kho_preserve_vmalloc(void *ptr, struct kho_vmalloc *preservation);
 int kho_unpreserve_vmalloc(struct kho_vmalloc *preservation);
+void *kho_alloc_preserve(size_t size);
+void kho_unpreserve_free(void *mem);
+void kho_restore_free(void *mem);
 struct folio *kho_restore_folio(phys_addr_t phys);
 struct page *kho_restore_pages(phys_addr_t phys, unsigned int nr_pages);
 void *kho_restore_vmalloc(const struct kho_vmalloc *preservation);
@@ -101,6 +105,14 @@ static inline int kho_unpreserve_vmalloc(struct kho_vmalloc *preservation)
 	return -EOPNOTSUPP;
 }
 
+static inline void *kho_alloc_preserve(size_t size)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
+
+static inline void kho_unpreserve_free(void *mem) { }
+static inline void kho_restore_free(void *mem) { }
+
 static inline struct folio *kho_restore_folio(phys_addr_t phys)
 {
 	return NULL;
@@ -122,18 +134,14 @@ static inline int kho_add_subtree(const char *name, void *fdt)
 	return -EOPNOTSUPP;
 }
 
-static inline void kho_remove_subtree(void *fdt)
-{
-}
+static inline void kho_remove_subtree(void *fdt) { }
 
 static inline int kho_retrieve_subtree(const char *name, phys_addr_t *phys)
 {
 	return -EOPNOTSUPP;
 }
 
-static inline void kho_memory_init(void)
-{
-}
+static inline void kho_memory_init(void) { }
 
 static inline void kho_populate(phys_addr_t fdt_phys, u64 fdt_len,
 				phys_addr_t scratch_phys, u64 scratch_len)