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

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

On 6/20/22 00:06, Sourabh Jain wrote:
> 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?
Sourabh,
Yes, you are correct. I'll add __weak versions of each in crash_core.c in the next patch.
Thanks!
Eric

> 
> Thanks,
> Sourabh Jain
> 

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