Re: [PATCH v9 7/7] x86/crash: Add x86 crash hotplug support

Sourabh Jain
2022-06-20T05:08:00Z

Hello Eric,

On 14/06/22 04:12, 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/kernel/crash.c | 116 ++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 127 insertions(+)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 762a0b6ab8b6..e9eecff3b97d 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2082,6 +2082,17 @@ 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..b9cdf7a0d868 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 +424,108 @@ int crash_load_segments(struct kimage *image)
> return ret;
> }
> #endif /* CONFIG_KEXEC_FILE */
> +
> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
> +int crash_hotplug_support(void) { return 1; }
> +void *arch_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;
> +}
> +
> +void arch_unmap_crash_pages(void **ptr)
> +{
> + if (ptr) {
> + if (*ptr)
> + kunmap(*ptr);
> + *ptr = NULL;
> + }
> +}

Aren't arch will have build issue if arch_[un]map_crash_pages methods
are not defined?

Thanks,
Sourabh Jain