riscv: crash: Add crash hotplug support

When CPUs or memory are hot-plugged on a running system, the crash
dump's elfcorehdr -- which describes the available CPUs and memory to
the capture kernel -- must be kept in sync, otherwise a crash after a
hotplug event would produce a vmcore that does not reflect the current
system topology.

Enable crash hotplug for RISC-V by selecting
ARCH_SUPPORTS_CRASH_HOTPLUG and providing the three architecture hooks
invoked through the override macros in <asm/kexec.h>:

  arch_crash_hotplug_support() reports whether an image supports the
  mechanism; arch_crash_get_elfcorehdr_size() sizes the segment for
  NR_CPUS and, with memory hotplug, CRASH_MAX_MEMORY_RANGES;
  arch_crash_handle_hotplug_event() rebuilds the elfcorehdr through the
  crash_prepare_headers() helper and writes it over the old segment.

The elfcorehdr is copied with memcpy_flushcache(), matching the x86
implementation; on RISC-V this currently expands to a plain memcpy()
and will benefit automatically once the architecture provides a
flushcache backend. The implementation otherwise follows the x86 crash
hotplug model.

Signed-off-by: Rui Qi <qirui.001@bytedance.com>
Tested-by: Rui Qi <qirui.001@bytedance.com>
Link: https://patch.msgid.link/20260910032619.2477279-1-qirui.001@bytedance.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

arch/riscv/Kconfig

@@ -1133,6 +1133,9 @@ config ARCH_DEFAULT_CRASH_DUMP
 config ARCH_HAS_GENERIC_CRASHKERNEL_RESERVATION
 	def_bool CRASH_RESERVE
 
+config ARCH_SUPPORTS_CRASH_HOTPLUG
+	def_bool y
+
 config COMPAT
 	bool "Kernel support for 32-bit U-mode"
 	default 64BIT

arch/riscv/include/asm/kexec.h

@@ -75,4 +75,17 @@ int load_extra_segments(struct kimage *image, unsigned long kernel_start,
 			unsigned long cmdline_len);
 #endif
 
+#ifdef CONFIG_CRASH_HOTPLUG
+struct kimage;
+
+void arch_crash_handle_hotplug_event(struct kimage *image, void *arg);
+#define arch_crash_handle_hotplug_event arch_crash_handle_hotplug_event
+
+int arch_crash_hotplug_support(struct kimage *image, unsigned long kexec_flags);
+#define arch_crash_hotplug_support arch_crash_hotplug_support
+
+unsigned int arch_crash_get_elfcorehdr_size(void);
+#define crash_get_elfcorehdr_size arch_crash_get_elfcorehdr_size
+#endif
+
 #endif

arch/riscv/kernel/Makefile

@@ -111,6 +111,7 @@ obj-$(CONFIG_KGDB)		+= kgdb.o
 obj-$(CONFIG_KEXEC_CORE)	+= kexec_relocate.o crash_save_regs.o machine_kexec.o
 obj-$(CONFIG_KEXEC_FILE)	+= kexec_elf.o kexec_image.o machine_kexec_file.o
 obj-$(CONFIG_CRASH_DUMP)	+= crash_dump.o
+obj-$(CONFIG_CRASH_HOTPLUG)	+= crash.o
 obj-$(CONFIG_VMCORE_INFO)	+= vmcore_info.o
 
 obj-$(CONFIG_JUMP_LABEL)	+= jump_label.o

arch/riscv/kernel/crash.c

@@ -0,0 +1,113 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * RISC-V crash hotplug support
+ *
+ * Copyright (C) 2025 Bytedance Ltd.
+ */
+
+#include <linux/kexec.h>
+#include <linux/crash_core.h>
+#include <linux/mm.h>
+#include <linux/vmalloc.h>
+
+#ifdef CONFIG_CRASH_HOTPLUG
+
+#undef pr_fmt
+#define pr_fmt(fmt) "crash hp: " fmt
+
+int arch_crash_hotplug_support(struct kimage *image, unsigned long kexec_flags)
+{
+#ifdef CONFIG_KEXEC_FILE
+	if (image->file_mode)
+		return 1;
+#endif
+	/*
+	 * For the kexec_load() syscall path, the user space kexec tool
+	 * needs to indicate that the elfcorehdr segment is excluded from
+	 * SHA verification by setting the appropriate flags.
+	 */
+	return (kexec_flags & KEXEC_UPDATE_ELFCOREHDR ||
+		kexec_flags & KEXEC_CRASH_HOTPLUG_SUPPORT);
+}
+
+unsigned int arch_crash_get_elfcorehdr_size(void)
+{
+	unsigned int sz;
+
+	/* kernel_map, VMCOREINFO and maximum CPUs */
+	sz = 2 + CONFIG_NR_CPUS;
+	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
+		sz += CONFIG_CRASH_MAX_MEMORY_RANGES;
+	sz *= sizeof(Elf64_Phdr);
+	return sz;
+}
+
+/**
+ * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
+ * @image: a pointer to kexec_crash_image
+ * @arg: struct memory_notify handler for memory hotplug case and
+ *       NULL for CPU hotplug case.
+ *
+ * Prepare the new elfcorehdr and replace the existing elfcorehdr.
+ */
+void arch_crash_handle_hotplug_event(struct kimage *image, void *arg)
+{
+	void *elfbuf = NULL, *old_elfcorehdr;
+	unsigned long mem, memsz;
+	unsigned long elfsz = 0;
+
+	/*
+	 * As crash_prepare_elf64_headers() has already described all
+	 * possible CPUs, there is no need to update the elfcorehdr
+	 * for additional CPU changes.
+	 */
+	if ((image->file_mode || image->elfcorehdr_updated) &&
+		((image->hp_action == KEXEC_CRASH_HP_ADD_CPU) ||
+		(image->hp_action == KEXEC_CRASH_HP_REMOVE_CPU)))
+		return;
+
+	/*
+	 * Create the new elfcorehdr reflecting the changes to CPU and/or
+	 * memory resources.
+	 */
+	if (crash_prepare_headers(true, &elfbuf, &elfsz, NULL)) {
+		pr_err("unable to create new elfcorehdr");
+		goto out;
+	}
+
+	/*
+	 * Obtain address and size of the elfcorehdr segment, and
+	 * check it against the new elfcorehdr buffer.
+	 */
+	mem = image->segment[image->elfcorehdr_index].mem;
+	memsz = image->segment[image->elfcorehdr_index].memsz;
+	if (elfsz > memsz) {
+		pr_err("update elfcorehdr elfsz %lu > memsz %lu",
+			elfsz, memsz);
+		goto out;
+	}
+
+	/*
+	 * Copy new elfcorehdr over the old elfcorehdr at destination.
+	 */
+	old_elfcorehdr = kmap_local_page(pfn_to_page(mem >> PAGE_SHIFT));
+	if (!old_elfcorehdr) {
+		pr_err("mapping elfcorehdr segment failed\n");
+		goto out;
+	}
+
+	/*
+	 * Temporarily invalidate the crash image while the
+	 * elfcorehdr is updated.
+	 */
+	xchg(&kexec_crash_image, NULL);
+	memcpy_flushcache(old_elfcorehdr, elfbuf, elfsz);
+	xchg(&kexec_crash_image, image);
+	kunmap_local(old_elfcorehdr);
+	pr_debug("updated elfcorehdr\n");
+
+out:
+	vfree(elfbuf);
+}
+
+#endif /* CONFIG_CRASH_HOTPLUG */