[PATCH v2 03/17] kexec: Add Kexec HandOver (KHO) generation helpers

From: Alexander Graf
Date: Fri Dec 22 2023 - 14:37:28 EST

This patch adds the core infrastructure to generate Kexec HandOver
metadata. Kexec HandOver is a mechanism that allows Linux to preserve
state - arbitrary properties as well as memory locations - across kexec.

It does so using 3 concepts:

1) Device Tree - Every KHO kexec carries a KHO specific flattened
device tree blob that describes the state of the system. Device
drivers can register to KHO to serialize their state before kexec.

2) Mem cache - A memblocks like structure that contains full page
ranges of reservations. These can not be part of the architectural
reservations, because they differ on every kexec.

3) Scratch Region - A CMA region that we allocate in the first kernel.
CMA gives us the guarantee that no handover pages land in that
region, because handover pages must be at a static physical memory
location. We use this region as the place to load future kexec
images into which then won't collide with any handover data.

Signed-off-by: Alexander Graf <graf@xxxxxxxxxx>

---

v1 -> v2:

- s/kho_reserve/kho_reserve_scratch/g
- Move kho enums out of ifdef
---
Documentation/ABI/testing/sysfs-kernel-kho | 53 +++
.../admin-guide/kernel-parameters.txt | 10 +
MAINTAINERS | 1 +
include/linux/kexec.h | 24 ++
include/uapi/linux/kexec.h | 6 +
kernel/Makefile | 1 +
kernel/kexec_kho_out.c | 316 ++++++++++++++++++
7 files changed, 411 insertions(+)
create mode 100644 Documentation/ABI/testing/sysfs-kernel-kho
create mode 100644 kernel/kexec_kho_out.c

diff --git a/Documentation/ABI/testing/sysfs-kernel-kho b/Documentation/ABI/testing/sysfs-kernel-kho
new file mode 100644
index 000000000000..f69e7b81a337
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-kernel-kho
@@ -0,0 +1,53 @@
+What: /sys/kernel/kho/active
+Date: December 2023
+Contact: Alexander Graf <graf@xxxxxxxxxx>
+Description:
+ Kexec HandOver (KHO) allows Linux to transition the state of
+ compatible drivers into the next kexec'ed kernel. To do so,
+ device drivers will serialize their current state into a DT.
+ While the state is serialized, they are unable to perform
+ any modifications to state that was serialized, such as
+ handed over memory allocations.
+
+ When this file contains "1", the system is in the transition
+ state. When contains "0", it is not. To switch between the
+ two states, echo the respective number into this file.
+
+What: /sys/kernel/kho/dt_max
+Date: December 2023
+Contact: Alexander Graf <graf@xxxxxxxxxx>
+Description:
+ KHO needs to allocate a buffer for the DT that gets
+ generated before it knows the final size. By default, it
+ will allocate 10 MiB for it. You can write to this file
+ to modify the size of that allocation.
+
+What: /sys/kernel/kho/scratch_len
+Date: December 2023
+Contact: Alexander Graf <graf@xxxxxxxxxx>
+Description:
+ To support continuous KHO kexecs, we need to reserve a
+ physically contiguous memory region that will always stay
+ available for future kexec allocations. This file describes
+ the length of that memory region. Kexec user space tooling
+ can use this to determine where it should place its payload
+ images.
+
+What: /sys/kernel/kho/scratch_phys
+Date: December 2023
+Contact: Alexander Graf <graf@xxxxxxxxxx>
+Description:
+ To support continuous KHO kexecs, we need to reserve a
+ physically contiguous memory region that will always stay
+ available for future kexec allocations. This file describes
+ the physical location of that memory region. Kexec user space
+ tooling can use this to determine where it should place its
+ payload images.
+
+What: /sys/kernel/kho/dt
+Date: December 2023
+Contact: Alexander Graf <graf@xxxxxxxxxx>
+Description:
+ When KHO is active, the kernel exposes the generated DT that
+ carries its current KHO state in this file. Kexec user space
+ tooling can use this as input file for the KHO payload image.
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 51575cd31741..efeef075617e 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2504,6 +2504,16 @@
kgdbwait [KGDB] Stop kernel execution and enter the
kernel debugger at the earliest opportunity.

+ kho_scratch=n[KMG] [KEXEC] Sets the size of the KHO scratch
+ region. The KHO scratch region is a physically
+ memory range that can only be used for non-kernel
+ allocations. That way, even when memory is heavily
+ fragmented with handed over memory, kexec will always
+ be able to find contiguous memory to place the next
+ kernel for kexec into.
+
+ The default is 0.
+
kmac= [MIPS] Korina ethernet MAC address.
Configure the RouterBoard 532 series on-chip
Ethernet adapter MAC address.
diff --git a/MAINTAINERS b/MAINTAINERS
index 9104430e148e..2a19bd282dd0 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -11713,6 +11713,7 @@ M: Eric Biederman <ebiederm@xxxxxxxxxxxx>
L: kexec@xxxxxxxxxxxxxxxxxxx
S: Maintained
W: http://kernel.org/pub/linux/utils/kernel/kexec/
+F: Documentation/ABI/testing/sysfs-kernel-kho
F: include/linux/kexec.h
F: include/uapi/linux/kexec.h
F: kernel/kexec*
diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index 8227455192b7..5d3b6b015838 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -21,6 +21,8 @@

#include <uapi/linux/kexec.h>
#include <linux/verification.h>
+#include <linux/libfdt.h>
+#include <linux/notifier.h>

extern note_buf_t __percpu *crash_notes;

@@ -516,6 +518,28 @@ void set_kexec_sig_enforced(void);
static inline void set_kexec_sig_enforced(void) {}
#endif

+/* Notifier index */
+enum kho_event {
+ KEXEC_KHO_DUMP = 0,
+ KEXEC_KHO_ABORT = 1,
+};
+
+#ifdef CONFIG_KEXEC_KHO
+extern phys_addr_t kho_scratch_phys;
+extern phys_addr_t kho_scratch_len;
+
+/* egest handover metadata */
+void kho_reserve_scratch(void);
+int register_kho_notifier(struct notifier_block *nb);
+int unregister_kho_notifier(struct notifier_block *nb);
+bool kho_is_active(void);
+#else
+static inline void kho_reserve_scratch(void) {}
+static inline int register_kho_notifier(struct notifier_block *nb) { return -EINVAL; }
+static inline int unregister_kho_notifier(struct notifier_block *nb) { return -EINVAL; }
+static inline bool kho_is_active(void) { return false; }
+#endif
+
#endif /* !defined(__ASSEBMLY__) */

#endif /* LINUX_KEXEC_H */
diff --git a/include/uapi/linux/kexec.h b/include/uapi/linux/kexec.h
index 01766dd839b0..d02ffd5960d6 100644
--- a/include/uapi/linux/kexec.h
+++ b/include/uapi/linux/kexec.h
@@ -49,6 +49,12 @@
/* The artificial cap on the number of segments passed to kexec_load. */
#define KEXEC_SEGMENT_MAX 16

+/* KHO passes an array of kho_mem as "mem cache" to the new kernel */
+struct kho_mem {
+ __u64 addr;
+ __u64 len;
+};
+
#ifndef __KERNEL__
/*
* This structure is used to hold the arguments that are used when
diff --git a/kernel/Makefile b/kernel/Makefile
index 3947122d618b..a6bd31e22c09 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -73,6 +73,7 @@ obj-$(CONFIG_KEXEC_CORE) += kexec_core.o
obj-$(CONFIG_KEXEC) += kexec.o
obj-$(CONFIG_KEXEC_FILE) += kexec_file.o
obj-$(CONFIG_KEXEC_ELF) += kexec_elf.o
+obj-$(CONFIG_KEXEC_KHO) += kexec_kho_out.o
obj-$(CONFIG_BACKTRACE_SELF_TEST) += backtracetest.o
obj-$(CONFIG_COMPAT) += compat.o
obj-$(CONFIG_CGROUPS) += cgroup/
diff --git a/kernel/kexec_kho_out.c b/kernel/kexec_kho_out.c
new file mode 100644
index 000000000000..765cf6ba7a46
--- /dev/null
+++ b/kernel/kexec_kho_out.c
@@ -0,0 +1,316 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * kexec_kho_out.c - kexec handover code to egest metadata.
+ * Copyright (C) 2023 Alexander Graf <graf@xxxxxxxxxx>
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/cma.h>
+#include <linux/kexec.h>
+#include <linux/device.h>
+#include <linux/compiler.h>
+#include <linux/kmsg_dump.h>
+
+struct kho_out {
+ struct kobject *kobj;
+ bool active;
+ struct cma *cma;
+ struct blocking_notifier_head chain_head;
+ void *dt;
+ u64 dt_len;
+ u64 dt_max;
+ struct mutex lock;
+};
+
+static struct kho_out kho = {
+ .dt_max = (1024 * 1024 * 10),
+ .chain_head = BLOCKING_NOTIFIER_INIT(kho.chain_head),
+ .lock = __MUTEX_INITIALIZER(kho.lock),
+};
+
+/*
+ * Size for scratch (non-KHO) memory. With KHO enabled, memory can become
+ * fragmented because KHO regions may be anywhere in physical address
+ * space. The scratch region gives us a safe zone that we will never see
+ * KHO allocations from. This is where we can later safely load our new kexec
+ * images into.
+ */
+static phys_addr_t kho_scratch_size __initdata;
+
+int register_kho_notifier(struct notifier_block *nb)
+{
+ return blocking_notifier_chain_register(&kho.chain_head, nb);
+}
+EXPORT_SYMBOL_GPL(register_kho_notifier);
+
+int unregister_kho_notifier(struct notifier_block *nb)
+{
+ return blocking_notifier_chain_unregister(&kho.chain_head, nb);
+}
+EXPORT_SYMBOL_GPL(unregister_kho_notifier);
+
+bool kho_is_active(void)
+{
+ return kho.active;
+}
+EXPORT_SYMBOL_GPL(kho_is_active);
+
+static ssize_t raw_read(struct file *file, struct kobject *kobj,
+ struct bin_attribute *attr, char *buf,
+ loff_t pos, size_t count)
+{
+ mutex_lock(&kho.lock);
+ memcpy(buf, attr->private + pos, count);
+ mutex_unlock(&kho.lock);
+
+ return count;
+}
+
+static BIN_ATTR(dt, 0400, raw_read, NULL, 0);
+
+static int kho_expose_dt(void *fdt)
+{
+ long fdt_len = fdt_totalsize(fdt);
+ int err;
+
+ kho.dt = fdt;
+ kho.dt_len = fdt_len;
+
+ bin_attr_dt.size = fdt_totalsize(fdt);
+ bin_attr_dt.private = fdt;
+ err = sysfs_create_bin_file(kho.kobj, &bin_attr_dt);
+
+ return err;
+}
+
+static void kho_abort(void)
+{
+ if (!kho.active)
+ return;
+
+ sysfs_remove_bin_file(kho.kobj, &bin_attr_dt);
+
+ kvfree(kho.dt);
+ kho.dt = NULL;
+ kho.dt_len = 0;
+
+ blocking_notifier_call_chain(&kho.chain_head, KEXEC_KHO_ABORT, NULL);
+
+ kho.active = false;
+}
+
+static int kho_serialize(void)
+{
+ void *fdt = NULL;
+ int err;
+
+ kho.active = true;
+ err = -ENOMEM;
+
+ fdt = kvmalloc(kho.dt_max, GFP_KERNEL);
+ if (!fdt)
+ goto out;
+
+ if (fdt_create(fdt, kho.dt_max)) {
+ err = -EINVAL;
+ goto out;
+ }
+
+ err = fdt_finish_reservemap(fdt);
+ if (err)
+ goto out;
+
+ err = fdt_begin_node(fdt, "");
+ if (err)
+ goto out;
+
+ err = fdt_property_string(fdt, "compatible", "kho-v1");
+ if (err)
+ goto out;
+
+ /* Loop through all kho dump functions */
+ err = blocking_notifier_call_chain(&kho.chain_head, KEXEC_KHO_DUMP, fdt);
+ err = notifier_to_errno(err);
+ if (err)
+ goto out;
+
+ /* Close / */
+ err = fdt_end_node(fdt);
+ if (err)
+ goto out;
+
+ err = fdt_finish(fdt);
+ if (err)
+ goto out;
+
+ if (WARN_ON(fdt_check_header(fdt))) {
+ err = -EINVAL;
+ goto out;
+ }
+
+ err = kho_expose_dt(fdt);
+
+out:
+ if (err) {
+ pr_err("kho failed to serialize state: %d", err);
+ kho_abort();
+ }
+ return err;
+}
+
+/* Handling for /sys/kernel/kho */
+
+#define KHO_ATTR_RO(_name) static struct kobj_attribute _name##_attr = __ATTR_RO_MODE(_name, 0400)
+#define KHO_ATTR_RW(_name) static struct kobj_attribute _name##_attr = __ATTR_RW_MODE(_name, 0600)
+
+static ssize_t active_store(struct kobject *dev, struct kobj_attribute *attr,
+ const char *buf, size_t size)
+{
+ ssize_t retsize = size;
+ bool val = false;
+ int ret;
+
+ if (kstrtobool(buf, &val) < 0)
+ return -EINVAL;
+
+ if (!kho_scratch_len)
+ return -ENOMEM;
+
+ mutex_lock(&kho.lock);
+ if (val != kho.active) {
+ if (val) {
+ ret = kho_serialize();
+ if (ret) {
+ retsize = -EINVAL;
+ goto out;
+ }
+ } else {
+ kho_abort();
+ }
+ }
+
+out:
+ mutex_unlock(&kho.lock);
+ return retsize;
+}
+
+static ssize_t active_show(struct kobject *dev, struct kobj_attribute *attr,
+ char *buf)
+{
+ ssize_t ret;
+
+ mutex_lock(&kho.lock);
+ ret = sysfs_emit(buf, "%d\n", kho.active);
+ mutex_unlock(&kho.lock);
+
+ return ret;
+}
+KHO_ATTR_RW(active);
+
+static ssize_t dt_max_store(struct kobject *dev, struct kobj_attribute *attr,
+ const char *buf, size_t size)
+{
+ u64 val;
+
+ if (kstrtoull(buf, 0, &val))
+ return -EINVAL;
+
+ kho.dt_max = val;
+
+ return size;
+}
+
+static ssize_t dt_max_show(struct kobject *dev, struct kobj_attribute *attr,
+ char *buf)
+{
+ return sysfs_emit(buf, "0x%llx\n", kho.dt_max);
+}
+KHO_ATTR_RW(dt_max);
+
+static ssize_t scratch_len_show(struct kobject *dev, struct kobj_attribute *attr,
+ char *buf)
+{
+ return sysfs_emit(buf, "0x%llx\n", kho_scratch_len);
+}
+KHO_ATTR_RO(scratch_len);
+
+static ssize_t scratch_phys_show(struct kobject *dev, struct kobj_attribute *attr,
+ char *buf)
+{
+ return sysfs_emit(buf, "0x%llx\n", kho_scratch_phys);
+}
+KHO_ATTR_RO(scratch_phys);
+
+static __init int kho_out_init(void)
+{
+ int ret = 0;
+
+ kho.kobj = kobject_create_and_add("kho", kernel_kobj);
+ if (!kho.kobj) {
+ ret = -ENOMEM;
+ goto err;
+ }
+
+ ret = sysfs_create_file(kho.kobj, &active_attr.attr);
+ if (ret)
+ goto err;
+
+ ret = sysfs_create_file(kho.kobj, &dt_max_attr.attr);
+ if (ret)
+ goto err;
+
+ ret = sysfs_create_file(kho.kobj, &scratch_phys_attr.attr);
+ if (ret)
+ goto err;
+
+ ret = sysfs_create_file(kho.kobj, &scratch_len_attr.attr);
+ if (ret)
+ goto err;
+
+err:
+ return ret;
+}
+late_initcall(kho_out_init);
+
+static int __init early_kho_scratch(char *p)
+{
+ kho_scratch_size = memparse(p, &p);
+ return 0;
+}
+early_param("kho_scratch", early_kho_scratch);
+
+/**
+ * kho_reserve_scratch - Reserve a contiguous chunk of memory for kexec
+ *
+ * With KHO we can preserve arbitrary pages in the system. To ensure we still
+ * have a large contiguous region of memory when we search the physical address
+ * space for target memory, let's make sure we always have a large CMA region
+ * active. This CMA region will only be used for movable pages which are not a
+ * problem for us during KHO because we can just move them somewhere else.
+ */
+__init void kho_reserve_scratch(void)
+{
+ int r;
+
+ if (kho_get_fdt()) {
+ /*
+ * We came from a previous KHO handover, so we already have
+ * a known good scratch region that we preserve. No need to
+ * allocate another.
+ */
+ return;
+ }
+
+ /* Only allocate KHO scratch memory when we're asked to */
+ if (!kho_scratch_size)
+ return;
+
+ r = cma_declare_contiguous_nid(0, kho_scratch_size, 0, PAGE_SIZE, 0,
+ false, "kho", &kho.cma, NUMA_NO_NODE);
+ if (WARN_ON(r))
+ return;
+
+ kho_scratch_phys = cma_get_base(kho.cma);
+ kho_scratch_len = cma_get_size(kho.cma);
+}
--
2.40.1




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