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

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On 5/25/22 00:25, Sourabh Jain wrote:
Hello Eric,

On 06/05/22 00:16, 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 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@xxxxxxxxxx>
---
  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;

Prepare_elf_header uses crash_prepare_elf64_headers function to generate elfcorehdr.
Since crash_prepare_elf64_headers is defined under CONFIG_KEXEC_FILE option we
will have build issues if CONFIG_KEXEC is enabled and CONFIG_KEXEC_FILE is disabled.

How about moving crash_prepare_elf64_headers function to kernel/kexec_core.c?

Yes, of course. I originally protected against this scenario with having the
CRASH_HOTPLUG dependent on KEXEC_FILE, but as we eliminated CRASH_HOTPLUG, your
suggestion above is on point.

Thanks!
eric


Thanks,
Sourabh Jain


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