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    Subject[PATCH v4 09/10] crash hp: Add x86 crash hotplug support for kexec_file_load
    Date
    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 if no errors occur, it is
    installed over the top of the existing elfcorehdr.

    In the patch 'crash hp: 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_HOTPLUG_ELFCOREHDR_SZ configure item.

    With this change, the kexec_file_load syscall (not kexec_load)
    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@oracle.com>
    ---
    arch/x86/kernel/crash.c | 92 +++++++++++++++++++++++++++++++++++++++++
    1 file changed, 92 insertions(+)

    diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
    index 9db41cce8d97..6215c51919db 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;

    +#ifdef CONFIG_CRASH_HOTPLUG
    + /* Ensure elfcorehdr segment large enough for hotplug changes */
    + kbuf.memsz = CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ;
    + /* For marking as usable to crash kernel */
    + image->elf_headers_sz = kbuf.memsz;
    + /* Record the index of the elfcorehdr segment */
    + image->elf_index = image->nr_segments;
    + image->elf_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 +424,84 @@ int crash_load_segments(struct kimage *image)
    return ret;
    }
    #endif /* CONFIG_KEXEC_FILE */
    +
    +#ifdef CONFIG_CRASH_HOTPLUG
    +/**
    + * arch_crash_hotplug_handler() - Handle hotplug elfcorehdr changes
    + * @image: the active struct kimage
    + * @hp_action: the hot un/plug action being handled
    + * @a: first parameter dependent upon hp_action
    + * @b: first parameter dependent upon hp_action
    + *
    + * 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 if no errors,
    + * 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
    + * CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ.
    + * 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_hotplug_handler(struct kimage *image,
    + unsigned int hp_action, unsigned long a, unsigned long b)
    +{
    + struct kexec_segment *ksegment;
    + unsigned char *ptr = NULL;
    + unsigned long elfsz = 0;
    + void *elfbuf = NULL;
    + unsigned long mem, memsz;
    +
    + /* Must have valid elfcorehdr index */
    + if (!image->elf_index_valid) {
    + pr_err("crash hp: unable to locate elfcorehdr segment");
    + goto out;
    + }
    +
    + ksegment = &image->segment[image->elf_index];
    + mem = ksegment->mem;
    + memsz = ksegment->memsz;
    +
    + /*
    + * Create the new elfcorehdr reflecting the changes to CPU and/or
    + * memory resources. The elfcorehdr segment memsz must be
    + * sufficiently large to accommodate increases due to hotplug
    + * activity. See CRASH_HOTPLUG_ELFCOREHDR_SZ.
    + */
    + if (prepare_elf_headers(image, &elfbuf, &elfsz)) {
    + pr_err("crash hp: unable to prepare elfcore headers");
    + goto out;
    + }
    + if (elfsz > memsz) {
    + pr_err("crash hp: update elfcorehdr elfsz %lu > memsz %lu",
    + elfsz, memsz);
    + goto out;
    + }
    +
    + /*
    + * At this point, we are all but assured of success.
    + * Copy new elfcorehdr into destination.
    + */
    + ptr = map_crash_pages(mem, memsz);
    + if (ptr) {
    + /* Temporarily invalidate the crash image while it is replaced */
    + xchg(&kexec_crash_image, NULL);
    + /* Write the new elfcorehdr into memory */
    + memcpy_flushcache((void *)ptr, elfbuf, elfsz);
    + /* The crash image is now valid once again */
    + xchg(&kexec_crash_image, image);
    + }
    + unmap_crash_pages((void **)&ptr);
    + pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
    +
    +out:
    + if (elfbuf)
    + vfree(elfbuf);
    +}
    +#endif /* CONFIG_CRASH_HOTPLUG */
    --
    2.27.0
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