[PATCH v8 6/7] x86/crash: Add x86 crash hotplug support for kexec_file_load

[Sourabh Jain]

Hello Eric,

On 06/05/22 00:16, 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.
>
> 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. When loading the crash kernel via kexec_file_load,
> the elfcorehdr is identified at load time in crash_load_segments().
>
> Signed-off-by: Eric DeVolder <eric.devolder at oracle.com>
> ---
>   arch/x86/Kconfig        |  11 ++++
>   arch/x86/kernel/crash.c | 117 ++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 128 insertions(+)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 4bed3abf444d..bf1201fe6981 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2072,6 +2072,17 @@ 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/kernel/crash.c b/arch/x86/kernel/crash.c
> index 9db41cce8d97..951ef365f0a7 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>
> @@ -398,7 +399,17 @@ 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);
> +	/* For marking as usable to crash kernel */
> +	image->elf_headers_sz = kbuf.memsz;
> +	/* Record the index of the elfcorehdr segment */
> +	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);
> @@ -413,3 +424,109 @@ int crash_load_segments(struct kimage *image)
>   	return ret;
>   }
>   #endif /* CONFIG_KEXEC_FILE */
> +
> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
> +static void *map_crash_pages(unsigned long paddr, unsigned long size)
> +{
> +	/*
> +	 * 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 *ptr = NULL;
> +
> +	/* NOTE: requires arch_kexec_[un]protect_crashkres() for write access */
> +	if (size > 0) {
> +		struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
> +
> +		ptr = kmap(page);
> +	}
> +
> +	return ptr;
> +}
> +
> +static void unmap_crash_pages(void **ptr)
> +{
> +	if (ptr) {
> +		if (*ptr)
> +			kunmap(*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
> + * @cpu: when KEXEC_CRASH_HP_ADD/REMOVE_CPU, the cpu affected
> + *
> + * To accurately reflect hot un/plug changes, the elfcorehdr (which
> + * is passed to the crash kernel via the elfcorehdr= parameter)
> + * must be updated with the new list of CPUs and memories. The new
> + * elfcorehdr is prepared in a kernel buffer, and then it is
> + * written on top of the existing/old elfcorehdr.
> + *
> + * For hotplug changes to elfcorehdr to work, two conditions are
> + * needed:
> + * First, the segment containing the elfcorehdr must be large enough
> + * to permit a growing number of resources. See the
> + * CONFIG_CRASH_MAX_MEMORY_RANGES description.
> + * Second, purgatory must explicitly exclude the elfcorehdr from the
> + * list of segments it checks (since the elfcorehdr changes and thus
> + * would require an update to purgatory itself to update the digest).
> + *
> + */
> +void arch_crash_handle_hotplug_event(struct kimage *image,
> +	unsigned int hp_action, unsigned int cpu)
> +{
> +	struct kexec_segment *ksegment;
> +	unsigned char *ptr = NULL;
> +	unsigned long elfsz = 0;
> +	void *elfbuf = NULL;
> +	unsigned long mem, memsz;
> +
> +	if (!image->elfcorehdr_index_valid) {
> +		pr_err("crash hp: unable to locate elfcorehdr segment");
> +		goto out;
> +	}
> +
> +	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;

Prepare_elf_header uses crash_prepare_elf64_headers function to generate elfcorehdr.
Since crash_prepare_elf64_headers is defined under CONFIG_KEXEC_FILE option we
will have build issues if CONFIG_KEXEC is enabled and CONFIG_KEXEC_FILE is disabled.

How about moving crash_prepare_elf64_headers function to kernel/kexec_core.c?
  
Thanks,
Sourabh Jain