On 01/10/22 at 02:57pm, 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.
>
> 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: kexec_file changes for crash hotplug support'
> 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.
>
> NOTE that this supports both kexec_load and kexec_file_load. Support
> for kexec_load is made possible by identifying the elfcorehdr segment
> at load time and updating it as previously described. However, it is
> the responsibility of the userspace kexec utility to ensure that:
> - the elfcorehdr segment is sufficiently large enough to accommodate
> hotplug changes, ala CRASH_HOTPLUG_ELFCOREHDR_SZ.
> - provides a purgatory that excludes the elfcorehdr from its list of
> run-time segments to check.
> These changes to the userspace kexec utility are not yet available.
>
> Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
> ---
> arch/x86/kernel/crash.c | 138 +++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 137 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index 9730c88530fc..d185137b33d4 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>
> @@ -265,7 +266,8 @@ static int prepare_elf_headers(struct kimage *image, void **addr,
> goto out;
>
> /* By default prepare 64bit headers */
> - ret = crash_prepare_elf64_headers(cmem, IS_ENABLED(CONFIG_X86_64), addr, sz);
> + ret = crash_prepare_elf64_headers(image, cmem,
> + IS_ENABLED(CONFIG_X86_64), addr, sz);
>
> out:
> vfree(cmem);
> @@ -397,7 +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;
I would define a default value for the size, meantime provide a Kconfig
option to allow user to customize.
> + /* 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);
> @@ -412,3 +424,127 @@ int crash_load_segments(struct kimage *image)
> return ret;
> }
> #endif /* CONFIG_KEXEC_FILE */
> +
> +#ifdef CONFIG_CRASH_HOTPLUG
These two helper function should be carved out into a separate patch as
a preparatory one. I am considering how to rearrange and split the
patches, will reply to cover letter.
> +void *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 unmap_crash_pages(void **ptr)
> +{
> + if (ptr) {
> + if (*ptr)
> + kunmap(*ptr);
> + *ptr = NULL;
> + }
> +}
> +
> +void arch_crash_hotplug_handler(struct kimage *image,
> + unsigned int hp_action, unsigned long a, unsigned long b)
> +{
> + /*
> + * 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.
> + *
> + * Due to the change to the elfcorehdr, purgatory must explicitly
> + * exclude the elfcorehdr from the list of segments it checks.
> + */
Please move this code comment to above function as kernel-doc if you
this it benefits the entire function. Otherwise should move them above
the code block they are explaining. For this place, I think moving them
to above arch_crash_hotplug_handler() is better.
> + struct kexec_segment *ksegment;
> + unsigned char *ptr = NULL;
> + unsigned long elfsz = 0;
> + void *elfbuf = NULL;
> + unsigned long mem, memsz;
> + unsigned int n;
> +
> + /*
> + * When the struct kimage is alloced, it is wiped to zero, so
> + * the elf_index_valid defaults to false. It is set on the
> + * kexec_file_load path, or here for kexec_load.
> + */
I think this kexec loading part should be taken out and post after this
whole patchset being accepted. At least, it's worth to put them in a
separate patch.
> + if (!image->elf_index_valid) {
> + for (n = 0; n < image->nr_segments; n++) {
> + mem = image->segment[n].mem;
> + memsz = image->segment[n].memsz;
> + ptr = map_crash_pages(mem, memsz);
> + if (ptr) {
> + /* The segment containing elfcorehdr */
> + if ((ptr[0] == 0x7F) &&
> + (ptr[1] == 'E') &&
> + (ptr[2] == 'L') &&
> + (ptr[3] == 'F')) {
Is this for safety checking later?
> + image->elf_index = (int)n;
> + image->elf_index_valid = true;
> + }
> + }
> + unmap_crash_pages((void **)&ptr);
> + }
> + }
> +
> + /* Must have valid elfcorehdr index */
> + if (!image->elf_index_valid) {
> + pr_err("crash hp: unable to locate elfcorehdr segment");
> + goto out;
> + }
> +
> + /*
> + * 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;
> + }
> + ksegment = &image->segment[image->elf_index];
> + memsz = ksegment->memsz;
> + 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.
> + */
> + ksegment = &image->segment[image->elf_index];
> + mem = ksegment->mem;
> + memsz = ksegment->memsz;
The ksegment and memsz have repeated assignment.
> + 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((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);
> +
> +//FIX??? somekind of cache flush perhaps?
You might mean memcpy_flushcache() on x86.
> +
> +out:
> + if (elfbuf)
> + vfree(elfbuf);
> +}
> +#endif /* CONFIG_CRASH_HOTPLUG */
> --
> 2.27.0
>
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