Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support

[Eric DeVolder <eric.devolder@oracle.com>]

On 9/19/22 02:06, Sourabh Jain wrote:
> 
> On 10/09/22 02:35, 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>
>> Acked-by: Baoquan He <bhe@redhat.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;
> 
> On PowerPC, while preparing the elf core header the memblock structure is used to prepare program 
> header for memory regions of elfcorehdr. Since the above arch specific hotplug handler gets invoked 
> when memory is marked offline (MEM_OFFLINE) which is before memblock structure gets updated so on 
> PowerPC the above handler may not work for memory hotplug case.
> 
> Just wondering which data structure is used to get the list of memory regions while preparing 
> program header for memory regions of elfcorehdr on other architectures?
> 
> Thanks,
> Sourabh Jain

I think your request to report the memory block address in comments of patch 3/7 "crash: add generic 
infrastructure" cover this scenario now.
Thanks,
eric