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

Eric DeVolder
2022-07-08T21:08:28Z

On 7/8/22 14:38, Fabio M. De Francesco wrote:
> On martedì 14 giugno 2022 00:42:40 CEST 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);
>> + }
>
> kmap_local_page() is preferred over kmap() which should be avoided in new
> code (please take a look at highmem.rst which I recently updated).
>
> Can you please check if you can easily avoid using kmap()? Otherwise, this
> code will be added to my (long) queue and changed one day or the other :-)
>
> Thanks,
>
> Fabio
Fabio,
I converted this to kmap_local_page()!
Thanks!
eric

>
>> +
>> + return ptr;
>> +}
>> +
>> +void arch_unmap_crash_pages(void **ptr)
>> +{
>> + if (ptr) {
>> + if (*ptr)
>> + kunmap(*ptr);
>> + *ptr = NULL;
>> + }
>> +}
>> +
>> +/**
>> + * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
>> + * @image: the active struct kimage
>> + * @hp_action: the hot un/plug action being handled
>> + * @cpu: when KEXEC_CRASH_HP_ADD/REMOVE_CPU, the cpu affected
>> + *
>> + * 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 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 the
>> + * CONFIG_CRASH_MAX_MEMORY_RANGES description.
>> + * 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_handle_hotplug_event(struct kimage *image,
>> + unsigned int hp_action, unsigned int cpu)
>> +{
>> + struct kexec_segment *ksegment;
>> + unsigned char *ptr = NULL;
>> + unsigned long elfsz = 0;
>> + void *elfbuf = NULL;
>> + unsigned long mem, memsz;
>> +
>> + /*
>> + * Elfcorehdr_index_valid checked in
> crash_core:handle_hotplug_event()
>> + */
>> + ksegment = &image->segment[image->elfcorehdr_index];
>> + mem = ksegment->mem;
>> + memsz = ksegment->memsz;
>> +
>> + /*
>> + * Create the new elfcorehdr reflecting the changes to CPU and/or
>> + * memory resources.
>> + */
>> + 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 = arch_map_crash_pages(mem, memsz);
>> + if (ptr) {
>> + /*
>> + * Temporarily invalidate the crash image while the
>> + * elfcorehdr is updated.
>> + */
>> + xchg(&kexec_crash_image, NULL);
>> + memcpy_flushcache((void *)ptr, elfbuf, elfsz);
>> + xchg(&kexec_crash_image, image);
>> + }
>> + arch_unmap_crash_pages((void **)&ptr);
>> + pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
>> +
>> +out:
>> + if (elfbuf)
>> + vfree(elfbuf);
>> +}
>> +#endif
>> --
>> 2.31.1
>>
>>
>
>
>
>