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.

For now it 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>
---
 fs/pstore/Kconfig      |   9 ++
 fs/pstore/Makefile     |   2 +
 fs/pstore/pstore_kho.c | 267 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 278 insertions(+)
 create mode 100644 fs/pstore/pstore_kho.c

diff --git a/fs/pstore/Kconfig b/fs/pstore/Kconfig
index 3acc38600cd1..70853799e211 100644
--- a/fs/pstore/Kconfig
+++ b/fs/pstore/Kconfig
@@ -81,6 +81,15 @@ 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 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..3bc679273c8d
--- /dev/null
+++ b/fs/pstore/pstore_kho.c
@@ -0,0 +1,267 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * KHO (Kexec Handover) backend for pstore
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/init.h>
+#include <linux/io.h>
+#include <linux/kexec_handover.h>
+#include <linux/libfdt.h>
+#include <linux/module.h>
+#include <linux/pstore.h>
+#include <linux/slab.h>
+#include <linux/unaligned.h>
+
+#define KHO_PSTORE_FDT_NAME	"pstore-kho"
+#define KHO_PSTORE_COMPAT	"pstore-kho-v1"
+#define KHO_PSTORE_DATA		"pstore_kho_record"
+
+static const size_t record_max_size = 1 << CONFIG_LOG_BUF_SHIFT;
+
+struct pstore_kho_record {
+	ssize_t			size;
+	struct timespec64	time;
+	int			count;
+	enum kmsg_dump_reason	reason;
+	unsigned int		part;
+	bool			compressed;
+	char			buf[];
+};
+
+struct pstore_kho_context {
+	struct pstore_info pstore;
+	bool read_done;
+};
+
+static struct pstore_kho_record *kho_data_in;
+static struct pstore_kho_record *kho_data_out;
+static void *fdt_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;
+
+	if (cxt->read_done)
+		return 0;
+
+	cxt->read_done = true;
+
+	if (!kho_data_in || !kho_data_in->size)
+		return 0;
+
+	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 = kho_data_in->time;
+	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)
+{
+	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: %lu\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 = record->time;
+	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 fdt_phys;
+	void *fdt_in;
+	const phys_addr_t *kho_data_phys;
+	int err, len;
+
+	err = kho_retrieve_subtree(KHO_PSTORE_FDT_NAME, &fdt_phys);
+	if (err) {
+		if (err != -ENOENT)
+			pr_err("failed to retrieve FDT %s from KHO: %d\n",
+			       KHO_PSTORE_FDT_NAME, err);
+		return;
+	}
+
+	fdt_in = phys_to_virt(fdt_phys);
+	err = fdt_node_check_compatible(fdt_in, 0, KHO_PSTORE_COMPAT);
+	if (err) {
+		pr_err("FDT '%s' is incompatible with '%s' [%d]\n",
+		       KHO_PSTORE_FDT_NAME, KHO_PSTORE_COMPAT, err);
+		return;
+	}
+
+	kho_data_phys = fdt_getprop(fdt_in, 0, KHO_PSTORE_DATA, &len);
+	if (!kho_data_phys || len != sizeof(*kho_data_phys)) {
+		pr_err("failed to find kho property %s\n", KHO_PSTORE_DATA);
+		return;
+	}
+
+	kho_data_in = phys_to_virt(get_unaligned(kho_data_phys));
+
+	pr_info("successfully restored preserved data\n");
+}
+
+static int __init kho_setup_outgoing(void)
+{
+	phys_addr_t header_ser_phys;
+	int err;
+	size_t total_size = sizeof(struct pstore_kho_record) + record_max_size;
+
+	kho_data_out = kho_alloc_preserve(total_size);
+	if (IS_ERR(kho_data_out)) {
+		pr_err("failed to allocate pstore kho data\n");
+		return PTR_ERR(kho_data_out);
+	}
+
+	fdt_out = kho_alloc_preserve(PAGE_SIZE);
+	if (IS_ERR(fdt_out)) {
+		pr_err("failed to allocate/preserve FDT memory\n");
+		err = PTR_ERR(fdt_out);
+		goto err_free_header;
+	}
+
+	err = fdt_create(fdt_out, PAGE_SIZE);
+	err |= fdt_finish_reservemap(fdt_out);
+	err |= fdt_begin_node(fdt_out, "");
+	err |= fdt_property_string(fdt_out, "compatible", KHO_PSTORE_COMPAT);
+
+	header_ser_phys = virt_to_phys(kho_data_out);
+	err |= fdt_property(fdt_out, KHO_PSTORE_DATA, &header_ser_phys,
+			    sizeof(header_ser_phys));
+	err |= fdt_end_node(fdt_out);
+	err |= fdt_finish(fdt_out);
+	if (err) {
+		pr_err("failed to configure fdt tree\n");
+		goto err_free_fdt;
+	}
+
+	err = kho_add_subtree(KHO_PSTORE_FDT_NAME, fdt_out);
+	if (err) {
+		pr_err("failed to add subtree\n");
+		goto err_free_fdt;
+	}
+
+	return 0;
+
+err_free_fdt:
+	kho_unpreserve_free(fdt_out);
+
+err_free_header:
+	kho_unpreserve_free(kho_data_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");
+		goto err_free_incoming;
+	}
+
+	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(fdt_out);
+	kho_unpreserve_free(fdt_out);
+
+	kho_unpreserve_free(kho_data_out);
+
+err_free_incoming:
+	if (kho_data_in)
+		kho_restore_free(kho_data_in);
+
+	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(fdt_out);
+	kho_unpreserve_free(fdt_out);
+
+	kho_unpreserve_free(kho_data_out);
+
+	if (kho_data_in)
+		kho_restore_free(kho_data_in);
+}
+module_exit(pstore_kho_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Pstore backend for dmesg preservation over kexec");
-- 
2.54.0.669.g59709faab0-goog

On 05-20 19:43, Michal Clapinski wrote:
> 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.
> 
> For now it supports preserving only 1 dmesg buffer.

"only" sounds like a limitation, but I do not think it is. If userspace 
is configured properly after every boot, it can back up the dmesg from 
the previous kernel instance if needed.

> 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>
> ---
>  fs/pstore/Kconfig      |   9 ++
>  fs/pstore/Makefile     |   2 +
>  fs/pstore/pstore_kho.c | 267 +++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 278 insertions(+)
>  create mode 100644 fs/pstore/pstore_kho.c
> 
> diff --git a/fs/pstore/Kconfig b/fs/pstore/Kconfig
> index 3acc38600cd1..70853799e211 100644
> --- a/fs/pstore/Kconfig
> +++ b/fs/pstore/Kconfig
> @@ -81,6 +81,15 @@ 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 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..3bc679273c8d
> --- /dev/null
> +++ b/fs/pstore/pstore_kho.c
> @@ -0,0 +1,267 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * KHO (Kexec Handover) backend for pstore
> + */

We need a proper Doc comment here explaining the benefits of using a 
KHO-based pstore. It has some very significant advantages, such as 
removing the need to manage a hardcoded memmap or rely on firmware 
support. Also, you can mention that pstore also provides the ability to 
preserve every single dmesg even after filesystem unmounts, without 
being limited by the console baud rate.

> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
> +#include <linux/init.h>
> +#include <linux/io.h>
> +#include <linux/kexec_handover.h>
> +#include <linux/libfdt.h>
> +#include <linux/module.h>
> +#include <linux/pstore.h>
> +#include <linux/slab.h>
> +#include <linux/unaligned.h>
> +
> +#define KHO_PSTORE_FDT_NAME	"pstore-kho"
> +#define KHO_PSTORE_COMPAT	"pstore-kho-v1"
> +#define KHO_PSTORE_DATA		"pstore_kho_record"
> +
> +static const size_t record_max_size = 1 << CONFIG_LOG_BUF_SHIFT;

The above should be added to include/linux/kho/abi/pstore.h

Also, there is no need to add an FDT subtree; it is much cleaner to add 
a sub-blob and use a C-struct directly. Let's add something like struct 
pstore_ser to the pstore ABI header to act as an anchor for everything 
this code references.

Additionally, what happens if CONFIG_LOG_BUF_SHIFT changes between two 
kexec'd kernels? Will this still work? If not, then the buffer size 
needs to be explicitly defined as part of the ABI.

[...]

Pleae review 
https://sashiko.dev/#/patchset/20260520174332.921186-1-mclapinski%40google.com 
review comments.

Pasha