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Subject: [PATCH v11 7/7] x86/crash: Add x86 crash hotplug support
Date:   Fri, 26 Aug 2022 13:37:04 -0400
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For x86_64, when CPU or memory is hot un/plugged, the crash
elfcorehdr, which describes the CPUs and memory in the system,
must also be updated.

When loading the crash kernel via kexec_load or kexec_file_load,
the elfcorehdr is identified at run time in
crash_core:handle_hotplug_event().

To update the elfcorehdr for x86_64, a new elfcorehdr must be
generated from the available CPUs and memory. The new elfcorehdr
is prepared into a buffer, and then installed over the top of
the existing elfcorehdr.

In the patch 'kexec: exclude elfcorehdr from the segment digest'
the need to update purgatory due to the change in elfcorehdr was
eliminated.  As a result, no changes to purgatory or boot_params
(as the elfcorehdr= kernel command line parameter pointer
remains unchanged and correct) are needed, just elfcorehdr.

To accommodate a growing number of resources via hotplug, the
elfcorehdr segment must be sufficiently large enough to accommodate
changes, see the CRASH_MAX_MEMORY_RANGES configure item.

With this change, crash hotplug for kexec_file_load syscall
is supported. The kexec_load is also supported, but also
requires a corresponding change to userspace kexec-tools.

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
---
 arch/x86/Kconfig             |  11 ++++
 arch/x86/include/asm/kexec.h |  20 +++++++
 arch/x86/kernel/crash.c      | 102 +++++++++++++++++++++++++++++++++++
 3 files changed, 133 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index f9920f1341c8..cdfc9b2fdf98 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2056,6 +2056,17 @@ config CRASH_DUMP
 	  (CONFIG_RELOCATABLE=y).
 	  For more details see Documentation/admin-guide/kdump/kdump.rst
 
+config CRASH_MAX_MEMORY_RANGES
+	depends on CRASH_DUMP && KEXEC_FILE && (HOTPLUG_CPU || MEMORY_HOTPLUG)
+	int
+	default 32768
+	help
+	  For the kexec_file_load path, specify the maximum number of
+	  memory regions, eg. as represented by the 'System RAM' entries
+	  in /proc/iomem, that the elfcorehdr buffer/segment can accommodate.
+	  This value is combined with NR_CPUS and multiplied by Elf64_Phdr
+	  size to determine the final buffer size.
+
 config KEXEC_JUMP
 	bool "kexec jump"
 	depends on KEXEC && HIBERNATION
diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
index a3760ca796aa..432073385b2d 100644
--- a/arch/x86/include/asm/kexec.h
+++ b/arch/x86/include/asm/kexec.h
@@ -212,6 +212,26 @@ typedef void crash_vmclear_fn(void);
 extern crash_vmclear_fn __rcu *crash_vmclear_loaded_vmcss;
 extern void kdump_nmi_shootdown_cpus(void);
 
+void *arch_map_crash_pages(unsigned long paddr, unsigned long size);
+#define arch_map_crash_pages arch_map_crash_pages
+
+void arch_unmap_crash_pages(void **ptr);
+#define arch_unmap_crash_pages arch_unmap_crash_pages
+
+void arch_crash_handle_hotplug_event(struct kimage *image,
+		unsigned int hp_action);
+#define arch_crash_handle_hotplug_event arch_crash_handle_hotplug_event
+
+#ifdef CONFIG_HOTPLUG_CPU
+static inline int crash_hotplug_cpu_support(void) { return 1; }
+#define crash_hotplug_cpu_support crash_hotplug_cpu_support
+#endif
+
+#ifdef CONFIG_MEMORY_HOTPLUG
+static inline int crash_hotplug_memory_support(void) { return 1; }
+#define crash_hotplug_memory_support crash_hotplug_memory_support
+#endif
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_X86_KEXEC_H */
diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index 9ceb93c176a6..8fc7d678ac72 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -25,6 +25,7 @@
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
 #include <linux/memblock.h>
+#include <linux/highmem.h>
 
 #include <asm/processor.h>
 #include <asm/hardirq.h>
@@ -397,7 +398,18 @@ int crash_load_segments(struct kimage *image)
 	image->elf_headers = kbuf.buffer;
 	image->elf_headers_sz = kbuf.bufsz;
 
+#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
+	/* Ensure elfcorehdr segment large enough for hotplug changes */
+	kbuf.memsz =
+		(CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES) *
+			sizeof(Elf64_Phdr);
+	/* Mark as usable to crash kernel, else crash kernel fails on boot */
+	image->elf_headers_sz = kbuf.memsz;
+	image->elfcorehdr_index = image->nr_segments;
+	image->elfcorehdr_index_valid = true;
+#else
 	kbuf.memsz = kbuf.bufsz;
+#endif
 	kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
 	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
 	ret = kexec_add_buffer(&kbuf);
@@ -412,3 +424,93 @@ int crash_load_segments(struct kimage *image)
 	return ret;
 }
 #endif /* CONFIG_KEXEC_FILE */
+
+#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
+/*
+ * NOTE: The addresses and sizes passed to this routine have
+ * already been fully aligned on page boundaries. There is no
+ * need for massaging the address or size.
+ */
+void *arch_map_crash_pages(unsigned long paddr, unsigned long size)
+{
+	void *ptr = NULL;
+
+	if (size > 0) {
+		struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
+
+		ptr = kmap_local_page(page);
+	}
+
+	return ptr;
+}
+
+void arch_unmap_crash_pages(void **ptr)
+{
+	if (ptr) {
+		if (*ptr)
+			kunmap_local(*ptr);
+		*ptr = NULL;
+	}
+}
+
+/**
+ * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
+ * @image: the active struct kimage
+ * @hp_action: the hot un/plug action being handled
+ *
+ * To accurately reflect hot un/plug changes, the new elfcorehdr
+ * is prepared in a kernel buffer, and then it is written on top
+ * of the existing/old elfcorehdr.
+ */
+void arch_crash_handle_hotplug_event(struct kimage *image,
+	unsigned int hp_action)
+{
+	struct kexec_segment *ksegment;
+	unsigned char *ptr = NULL;
+	unsigned long elfsz = 0;
+	void *elfbuf = NULL;
+	unsigned long mem, memsz;
+
+	/*
+	 * Elfcorehdr_index_valid checked in crash_core:handle_hotplug_event()
+	 */
+	ksegment = &image->segment[image->elfcorehdr_index];
+	mem = ksegment->mem;
+	memsz = ksegment->memsz;
+
+	/*
+	 * Create the new elfcorehdr reflecting the changes to CPU and/or
+	 * memory resources.
+	 */
+	if (prepare_elf_headers(image, &elfbuf, &elfsz)) {
+		pr_err("crash hp: unable to prepare elfcore headers");
+		goto out;
+	}
+	if (elfsz > memsz) {
+		pr_err("crash hp: update elfcorehdr elfsz %lu > memsz %lu",
+			elfsz, memsz);
+		goto out;
+	}
+
+	/*
+	 * At this point, we are all but assured of success.
+	 * Copy new elfcorehdr into destination.
+	 */
+	ptr = arch_map_crash_pages(mem, memsz);
+	if (ptr) {
+		/*
+		 * Temporarily invalidate the crash image while the
+		 * elfcorehdr is updated.
+		 */
+		xchg(&kexec_crash_image, NULL);
+		memcpy_flushcache((void *)ptr, elfbuf, elfsz);
+		xchg(&kexec_crash_image, image);
+	}
+	arch_unmap_crash_pages((void **)&ptr);
+	pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
+
+out:
+	if (elfbuf)
+		vfree(elfbuf);
+}
+#endif
-- 
2.31.1