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    SubjectRe: [PATCH v15 7/7] x86/crash: add x86 crash hotplug support
    On 12/09/22 at 10:36am, Eric DeVolder wrote:
    ...... snip out
    > diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
    > index 9ceb93c176a6..5186df48ce6c 100644
    > --- a/arch/x86/kernel/crash.c
    > +++ b/arch/x86/kernel/crash.c
    > @@ -42,6 +42,21 @@
    > #include <asm/crash.h>
    > #include <asm/cmdline.h>
    >
    > +/*
    > + * For the kexec_file_load() syscall path, specify the maximum number of
    > + * memory regions that the elfcorehdr buffer/segment can accommodate.
    > + * These regions are obtained via walk_system_ram_res(); eg. the
    > + * 'System RAM' entries in /proc/iomem.
    > + * This value is combined with NR_CPUS_DEFAULT and multiplied by
    > + * sizeof(Elf64_Phdr) to determine the final elfcorehdr memory buffer/
    > + * segment size.
    > + * The value 8192, for example, covers a (sparsely populated) 1TiB system
    > + * consisting of 128MiB memblocks, while resulting in an elfcorehdr
    > + * memory buffer/segment size under 1MiB. This represents a sane choice
    > + * to accommodate both baremetal and virtual machine configurations.
    > + */
    > +#define CRASH_MAX_MEMORY_RANGES 8192
    > +
    > /* Used while preparing memory map entries for second kernel */
    > struct crash_memmap_data {
    > struct boot_params *params;
    > @@ -394,10 +409,39 @@ int crash_load_segments(struct kimage *image)
    > if (ret)
    > return ret;
    >
    > - image->elf_headers = kbuf.buffer;
    > - image->elf_headers_sz = kbuf.bufsz;
    > + image->elf_headers = kbuf.buffer;
    > + image->elf_headers_sz = kbuf.bufsz;
    > + kbuf.memsz = kbuf.bufsz;
    > +
    > +#ifdef CONFIG_CRASH_HOTPLUG
    > + if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {

    Do I miss anything important during reviewing? I can't see why memory
    hotplug is also relying on HOTPLUG_CPU.

    > + /*
    > + * Ensure the elfcorehdr segment large enough for hotplug changes.
    > + * Start with VMCOREINFO and kernel_map.
    > + */
    > + unsigned long pnum = 2;
    > +
    > + if (IS_ENABLED(CONFIG_HOTPLUG_CPU))
    > + pnum += CONFIG_NR_CPUS_DEFAULT;
    > +
    > + if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
    > + pnum += CRASH_MAX_MEMORY_RANGES;
    > +
    > + if (pnum < (unsigned long)PN_XNUM) {
    > + kbuf.memsz = pnum * sizeof(Elf64_Phdr);
    > + kbuf.memsz += sizeof(Elf64_Ehdr);
    > +
    > + image->elfcorehdr_index = image->nr_segments;
    > + image->elfcorehdr_index_valid = true;
    > +
    > + /* Mark as usable to crash kernel, else crash kernel fails on boot */
    > + image->elf_headers_sz = kbuf.memsz;
    > + } else {
    > + pr_err("number of Phdrs %lu exceeds max\n", pnum);
    > + }
    > + }
    > +#endif
    >
    > - kbuf.memsz = kbuf.bufsz;
    > kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
    > kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
    > ret = kexec_add_buffer(&kbuf);
    > @@ -412,3 +456,67 @@ int crash_load_segments(struct kimage *image)
    > return ret;
    > }
    > #endif /* CONFIG_KEXEC_FILE */
    > +
    > +#ifdef CONFIG_CRASH_HOTPLUG
    > +
    > +#undef pr_fmt
    > +#define pr_fmt(fmt) "crash hp: " fmt
    > +
    > +/**
    > + * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
    > + * @image: the active struct kimage
    > + *
    > + * 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)
    > +{
    > + void *elfbuf = NULL, *old_elfcorehdr;
    > + unsigned long mem, memsz;
    > + unsigned long elfsz = 0;
    > +
    > + /*
    > + * Create the new elfcorehdr reflecting the changes to CPU and/or
    > + * memory resources.
    > + */
    > + if (prepare_elf_headers(image, &elfbuf, &elfsz)) {
    > + pr_err("unable to prepare elfcore headers");
    > + goto out;
    > + }
    > +
    > + /*
    > + * Obtain address and size of the elfcorehdr segment, and
    > + * check it against the new elfcorehdr buffer.
    > + */
    > + mem = image->segment[image->elfcorehdr_index].mem;
    > + memsz = image->segment[image->elfcorehdr_index].memsz;
    > + if (elfsz > memsz) {
    > + pr_err("update elfcorehdr elfsz %lu > memsz %lu",
    > + elfsz, memsz);
    > + goto out;
    > + }
    > +
    > + /*
    > + * Copy new elfcorehdr over the old elfcorehdr at destination.
    > + */
    > + old_elfcorehdr = kmap_local_page(pfn_to_page(mem >> PAGE_SHIFT));
    > + if (old_elfcorehdr) {
    > + pr_err("updating elfcorehdr failed\n");
    > + goto out;
    > + }
    > +
    > + /*
    > + * Temporarily invalidate the crash image while the
    > + * elfcorehdr is updated.
    > + */
    > + xchg(&kexec_crash_image, NULL);
    > + memcpy_flushcache(old_elfcorehdr, elfbuf, elfsz);
    > + xchg(&kexec_crash_image, image);
    > + kunmap_local(old_elfcorehdr);
    > + pr_debug("updated elfcorehdr\n");
    > +
    > +out:
    > + vfree(elfbuf);
    > +}
    > +#endif
    > --
    > 2.31.1
    >

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