Up to this point to preserve late shutdown logs in memory, users had to
predefine a memory region using ramoops. This commit changes this by
preserving a buffer using kexec-handover.

pstore_kho supports preserving only 1 dmesg buffer.
It gets replaced with the new buffer on every kexec, so the user has to
copy the file out of pstore after every kexec.
There is no erase() support.

Signed-off-by: Michal Clapinski <mclapinski@google.com>
---
v2:
- Added a comment explaining the benefits of pstore_kho.
- Created include/linux/kho/abi/pstore.h.
- Got rid of the KHO subtree.
- Made sure never to free incoming kho data.
  This way the module can be safely reloaded.
- Sashiko complained that I trust the data coming from the old kernel.
  I ignored it. LMK if I shouldn't trust the old kernel.
---
 fs/pstore/Kconfig              |  10 ++
 fs/pstore/Makefile             |   2 +
 fs/pstore/pstore_kho.c         | 230 +++++++++++++++++++++++++++++++++
 include/linux/kho/abi/pstore.h |  27 ++++
 4 files changed, 269 insertions(+)
 create mode 100644 fs/pstore/pstore_kho.c
 create mode 100644 include/linux/kho/abi/pstore.h

diff --git a/fs/pstore/Kconfig b/fs/pstore/Kconfig
index 3acc38600cd1..455790fec955 100644
--- a/fs/pstore/Kconfig
+++ b/fs/pstore/Kconfig
@@ -81,6 +81,16 @@ config PSTORE_RAM
 
 	  For more information, see Documentation/admin-guide/ramoops.rst.
 
+config PSTORE_KHO
+	tristate "Preserve logs over kexec"
+	depends on PSTORE
+	depends on KEXEC_HANDOVER
+	help
+	  A pstore backend for preserving dmesg over KHO (kexec handover).
+	  It does not require any additional cmdline params to work.
+
+	  It supports preservation of only 1 dmesg file.
+
 config PSTORE_ZONE
 	tristate
 	depends on PSTORE
diff --git a/fs/pstore/Makefile b/fs/pstore/Makefile
index c270467aeece..518cd408bf8e 100644
--- a/fs/pstore/Makefile
+++ b/fs/pstore/Makefile
@@ -13,6 +13,8 @@ pstore-$(CONFIG_PSTORE_PMSG)	+= pmsg.o
 ramoops-objs += ram.o ram_core.o
 obj-$(CONFIG_PSTORE_RAM)	+= ramoops.o
 
+obj-$(CONFIG_PSTORE_KHO)	+= pstore_kho.o
+
 pstore_zone-objs += zone.o
 obj-$(CONFIG_PSTORE_ZONE)	+= pstore_zone.o
 
diff --git a/fs/pstore/pstore_kho.c b/fs/pstore/pstore_kho.c
new file mode 100644
index 000000000000..6d4187d91642
--- /dev/null
+++ b/fs/pstore/pstore_kho.c
@@ -0,0 +1,230 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KHO (Kexec Handover) backend for pstore.
+ *
+ * KHO-based pstore provides a mechanism to hand over pstore data (specifically
+ * dmesg logs) from one kernel to another across a kexec reboot using the
+ * Kexec Handover (KHO) framework.
+ *
+ * Key advantages of KHO-based pstore include:
+ * - No hardcoded memmap: Unlike ramoops, it does not require reserving a static
+ *   memory region in the bootloader or device tree. Memory is allocated
+ *   dynamically and handed over to the next kernel.
+ * - Firmware independence: It does not rely on platform firmware support (like
+ *   ACPI ERST or UEFI variable storage) to preserve logs across reboots.
+ * - High throughput: It avoids the performance bottlenecks of serial consoles,
+ *   not being limited by console baud rates.
+ * - Complete log preservation: It preserves all dmesg logs, including those
+ *   generated late in the reboot cycle after filesystems have been unmounted,
+ *   up to the point of the kexec jump.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/kho/abi/pstore.h>
+#include <linux/init.h>
+#include <linux/io.h>
+#include <linux/kexec_handover.h>
+#include <linux/module.h>
+#include <linux/pstore.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+
+/*
+ * The in and out buffers are separate and they need not be the same size.
+ * Therefore, this is not part of ABI.
+ */
+#define RECORD_MAX_SIZE		(1 << CONFIG_LOG_BUF_SHIFT)
+
+struct pstore_kho_context {
+	struct pstore_info pstore;
+	bool read_done;
+};
+
+static struct pstore_ser *kho_ser_in;
+static struct pstore_ser *kho_ser_out;
+
+static int pstore_kho_open(struct pstore_info *psi)
+{
+	struct pstore_kho_context *cxt = psi->data;
+
+	cxt->read_done = false;
+	return 0;
+}
+
+static ssize_t pstore_kho_read(struct pstore_record *record)
+{
+	struct pstore_kho_context *cxt = record->psi->data;
+	struct pstore_kho_record *kho_data_in;
+
+	if (cxt->read_done || !kho_ser_in)
+		return 0;
+
+	cxt->read_done = true;
+	kho_data_in = &kho_ser_in->record;
+
+	record->buf = kmemdup(kho_data_in->buf, kho_data_in->size, GFP_KERNEL);
+	if (!record->buf)
+		return -ENOMEM;
+
+	record->type = PSTORE_TYPE_DMESG;
+	record->id = 0;
+	record->size = kho_data_in->size;
+	record->time.tv_sec = kho_data_in->time_sec;
+	record->time.tv_nsec = kho_data_in->time_nsec;
+	record->count = kho_data_in->count;
+	record->reason = kho_data_in->reason;
+	record->part = kho_data_in->part;
+	record->compressed = kho_data_in->compressed;
+
+	return record->size;
+}
+
+static int pstore_kho_write(struct pstore_record *record)
+{
+	struct pstore_kho_record *kho_data_out = &kho_ser_out->record;
+
+	if (record->type != PSTORE_TYPE_DMESG)
+		return -EINVAL;
+
+	if (kho_data_out->size != 0) {
+		pr_err("pstore kho already contains a record\n");
+		return -ENOSPC;
+	}
+
+	if (record->size > RECORD_MAX_SIZE) {
+		pr_err("dmesg record too big, record size: %lu, available space: %d\n",
+		       record->size, RECORD_MAX_SIZE);
+		return -ENOSPC;
+	}
+
+	memcpy(kho_data_out->buf, record->buf, record->size);
+	kho_data_out->size = record->size;
+	kho_data_out->time_sec = record->time.tv_sec;
+	kho_data_out->time_nsec = record->time.tv_nsec;
+	kho_data_out->count = record->count;
+	kho_data_out->reason = record->reason;
+	kho_data_out->part = record->part;
+	kho_data_out->compressed = record->compressed;
+
+	return 0;
+}
+
+static struct pstore_kho_context pstore_kho_cxt = {
+	.pstore = {
+		.owner		= THIS_MODULE,
+		.name		= "kho",
+		.bufsize	= RECORD_MAX_SIZE,
+		.flags		= PSTORE_FLAGS_DMESG,
+		.max_reason	= KMSG_DUMP_SHUTDOWN,
+		.open		= pstore_kho_open,
+		.read		= pstore_kho_read,
+		.write		= pstore_kho_write,
+	},
+};
+
+static void __init kho_setup_incoming(void)
+{
+	phys_addr_t kho_ser_phys;
+	int err;
+
+	err = kho_retrieve_subtree(KHO_PSTORE_FDT_NAME, &kho_ser_phys);
+	if (err) {
+		if (err != -ENOENT)
+			pr_err("failed to retrieve KHO data %s: %d\n",
+			       KHO_PSTORE_FDT_NAME, err);
+		return;
+	}
+
+	kho_ser_in = phys_to_virt(kho_ser_phys);
+
+	if (kho_ser_in->version != KHO_PSTORE_VERSION) {
+		pr_err("unsupported KHO pstore version: %d\n", kho_ser_in->version);
+		kho_ser_in = NULL;
+		return;
+	}
+
+	pr_info("successfully restored preserved data\n");
+}
+
+static int __init kho_setup_outgoing(void)
+{
+	int err;
+	size_t total_size = sizeof(struct pstore_ser) + RECORD_MAX_SIZE;
+
+	kho_ser_out = kho_alloc_preserve(total_size);
+	if (IS_ERR(kho_ser_out)) {
+		pr_err("failed to allocate pstore kho ser anchor\n");
+		return PTR_ERR(kho_ser_out);
+	}
+	memset(kho_ser_out, 0, total_size);
+	kho_ser_out->version = KHO_PSTORE_VERSION;
+
+	err = kho_add_subtree(KHO_PSTORE_FDT_NAME, kho_ser_out);
+	if (err) {
+		pr_err("failed to add KHO data\n");
+		goto err_free_ser;
+	}
+
+	return 0;
+
+err_free_ser:
+	kho_unpreserve_free(kho_ser_out);
+	return err;
+}
+
+static int __init pstore_kho_init(void)
+{
+	int err;
+	struct pstore_kho_context *cxt = &pstore_kho_cxt;
+
+	if (!kho_is_enabled()) {
+		pr_info("KHO is disabled, pstore_kho cannot start\n");
+		return -ENODEV;
+	}
+
+	kho_setup_incoming();
+	err = kho_setup_outgoing();
+	if (err) {
+		pr_err("failed to setup outgoing KHO\n");
+		return err;
+	}
+
+	cxt->pstore.data = cxt;
+	cxt->pstore.buf = kmalloc(cxt->pstore.bufsize, GFP_KERNEL);
+	if (!cxt->pstore.buf) {
+		err = -ENOMEM;
+		goto err_free_outgoing;
+	}
+
+	err = pstore_register(&cxt->pstore);
+	if (err) {
+		pr_err("failed to register with pstore\n");
+		goto err_free_pstore_buf;
+	}
+
+	return 0;
+
+err_free_pstore_buf:
+	kfree(cxt->pstore.buf);
+
+err_free_outgoing:
+	kho_remove_subtree(kho_ser_out);
+	kho_unpreserve_free(kho_ser_out);
+
+	return err;
+}
+module_init(pstore_kho_init);
+
+static void __exit pstore_kho_exit(void)
+{
+	pstore_unregister(&pstore_kho_cxt.pstore);
+	kfree(pstore_kho_cxt.pstore.buf);
+
+	kho_remove_subtree(kho_ser_out);
+	kho_unpreserve_free(kho_ser_out);
+}
+module_exit(pstore_kho_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Pstore backend for dmesg preservation over kexec");
diff --git a/include/linux/kho/abi/pstore.h b/include/linux/kho/abi/pstore.h
new file mode 100644
index 000000000000..743ec64d67fc
--- /dev/null
+++ b/include/linux/kho/abi/pstore.h
@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _LINUX_KHO_ABI_PSTORE_H
+#define _LINUX_KHO_ABI_PSTORE_H
+
+#include <linux/types.h>
+
+#define KHO_PSTORE_FDT_NAME	"pstore-kho"
+#define KHO_PSTORE_VERSION	1
+
+struct pstore_kho_record {
+	s64			size;
+	s64			time_sec;
+	u32			time_nsec;
+	s32			count;
+	u32			reason;
+	u32			part;
+	u32			compressed;
+	char			buf[];
+};
+
+struct pstore_ser {
+	u32			version;
+	struct pstore_kho_record record;
+};
+
+#endif /* _LINUX_KHO_ABI_PSTORE_H */
-- 
2.54.0.1032.g2f8565e1d1-goog