Re: [PATCH v14 7/7] x86/crash: add x86 crash hotplug support

Borislav Petkov
2022-12-07T10:20:06Z

On Wed, Nov 16, 2022 at 04:46:43PM -0500, Eric DeVolder wrote:
> When CPU or memory is hot un/plugged, the crash elfcorehdr, which
> describes the CPUs and memory in the system, must also be updated.
>
> A new elfcorehdr is generated from the available CPUs and memory
> into a buffer, and then installed over the top of the existing
> elfcorehdr. The segment containing the elfcorehdr is identified
> at run time in crash_core:handle_hotplug_event(), which works for
> both the kexec_load() and kexec_file_load() syscalls.
>
> 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 description.
>
> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> ---
> arch/x86/Kconfig | 9 +++
> arch/x86/include/asm/kexec.h | 15 +++++
> arch/x86/kernel/crash.c | 106 ++++++++++++++++++++++++++++++++++-
> 3 files changed, 127 insertions(+), 3 deletions(-)

Some quick cleanups ontop, there's potential for more:

---
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index ffee99046942..486509030d3a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2091,13 +2091,16 @@ config CRASH_DUMP
For more details see Documentation/admin-guide/kdump/kdump.rst

config CRASH_HOTPLUG
- bool "kernel updates of crash elfcorehdr"
+ bool "Update the crash elfcorehdr on system configuration changes"
default n
depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG)
help
- Enable the kernel to directly update the crash elfcorehdr (which
- contains the list of CPUs and memory regions to be dumped upon
- a crash) in response to hot plug/unplug of CPUs or memory.
+ Enable direct updates to the crash elfcorehdr (which contains
+ the list of CPUs and memory regions to be dumped upon a crash)
+ in response to hot plug/unplug of CPUs or memory. This is a much
+ more advanced approach than userspace attempting that.
+
+ If unsure, say Y.

config KEXEC_JUMP
bool "kexec jump"
diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index d2238bcf8106..d26f208e582e 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -413,25 +413,32 @@ int crash_load_segments(struct kimage *image)
image->elf_headers_sz = kbuf.bufsz;
kbuf.memsz = kbuf.bufsz;

-#ifdef CONFIG_CRASH_HOTPLUG
- /* Ensure elfcorehdr segment large enough for hotplug changes */
- unsigned long pnum = 2; /* VMCOREINFO and kernel_map */
-
- 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);
+ if (IS_ENABLED(CONFIG_CRASH_HOTPLUG)) {
+ /*
+ * 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.buf_align = ELF_CORE_HEADER_ALIGN;
kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
@@ -463,10 +470,9 @@ int crash_load_segments(struct kimage *image)
*/
void arch_crash_handle_hotplug_event(struct kimage *image)
{
+ void *elfbuf, *old_elfcorehdr;
unsigned long mem, memsz;
- unsigned long elfsz = 0;
- void *elfbuf = NULL;
- void *ptr;
+ unsigned long elfsz;

/*
* Create the new elfcorehdr reflecting the changes to CPU and/or
@@ -489,26 +495,24 @@ void arch_crash_handle_hotplug_event(struct kimage *image)
goto out;
}

- /*
- * Copy new elfcorehdr over the old elfcorehdr at 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(ptr, elfbuf, elfsz);
- xchg(&kexec_crash_image, image);
- arch_unmap_crash_pages(ptr);
- pr_debug("updated elfcorehdr\n");
- } else {
+ /* Copy new elfcorehdr over the old elfcorehdr at destination. */
+ old_elfcorehdr = arch_map_crash_pages(mem, memsz);
+ 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);
+ arch_unmap_crash_pages(old_elfcorehdr);
+ pr_debug("updated elfcorehdr\n");
+
out:
- if (elfbuf)
- vfree(elfbuf);
+ vfree(elfbuf);
}
#endif

--
Regards/Gruss,
Boris.