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

[Eric DeVolder]

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@xxxxxxxxxx>
> > ---
> >   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