[PATCH v8 6/7] x86/crash: Add x86 crash hotplug support for kexec_file_load

Eric DeVolder
2022-05-05T18:48:14Z

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 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. 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/Kconfig | 11 ++++
arch/x86/kernel/crash.c | 117 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 128 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 4bed3abf444d..bf1201fe6981 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2072,6 +2072,17 @@ 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..951ef365f0a7 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 +424,109 @@ int crash_load_segments(struct kimage *image)
return ret;
}
#endif /* CONFIG_KEXEC_FILE */
+
+#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
+static 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;
+}
+
+static void 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;
+
+ if (!image->elfcorehdr_index_valid) {
+ pr_err("crash hp: unable to locate elfcorehdr segment");
+ goto out;
+ }
+
+ 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 = 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);
+ }
+ unmap_crash_pages((void **)&ptr);
+ pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
+
+out:
+ if (elfbuf)
+ vfree(elfbuf);
+}
+#endif
--
2.27.0