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

On 08/26/22 at 01:37pm, Eric DeVolder wrote:
> 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

Check these two functions again, wonder whether this is a common usage
to show support of a feature. I don't remember where we do like this. We
usually define a global variable to mark it? Anyway, it works. See if
other people have comments, I could be ignorant.

Other than this tiny concern, this patch looks good to me:

Acked-by: Baoquan He <bhe@redhat.com>

> +
>  #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
>

On 9/4/22 22:19, Baoquan He wrote:
> On 08/26/22 at 01:37pm, Eric DeVolder wrote:
>> 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
> 
> Check these two functions again, wonder whether this is a common usage
> to show support of a feature. I don't remember where we do like this. We
> usually define a global variable to mark it? Anyway, it works. See if
> other people have comments, I could be ignorant.

These functions are for use in generating the value of the associated the sysfs attribute.

The default-value function (which previously was a __weak function) is in include/linux/kexec.h and 
the arch-specific override in arch/<arch>/include/asm/kexec.h. This default-value function sets the 
return to 0, for not supported.

I looked around a bit, and don't necessarily see a precedent. Then again some of this conversion of 
away from __weak is still making its way. I did briefly experiment with using a variable but decided 
there would still need to be a similar function needed (mostly likely on the subsys_initcall) in 
order to properly set the variable. So with the move away from __weak, it seemed to make sense to 
just keep the individual functions as they are now, in the spirit of the move away from __weak.

If the variable is still desired, please let me know. I'll post v12 at the end of the week to 
incorporate the fixes for some of the nits previously identified!

> 
> Other than this tiny concern, this patch looks good to me:
> 
> Acked-by: Baoquan He <bhe@redhat.com>

Thank you!
eric

> 
>> +
>>   #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
>>
>