[PATCH v2 6/8] kexec: avoid effectively open-coding xzalloc_flex_struct()

[Jan Beulich <jbeulich@xxxxxxxx>]

There is a difference in generated code: xzalloc_bytes() forces
SMP_CACHE_BYTES alignment. But if code really cared about such higher
than default alignment, it should request so explicitly rather than
using a type-unsafe interface. And if e.g. cache line sharing was a
concern, the allocator itself should arrange to avoid such.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/kexec.c
+++ b/xen/common/kexec.c
@@ -463,7 +463,10 @@ static void * alloc_from_crash_heap(cons
 /* Allocate a crash note buffer for a newly onlined cpu. */
 static int kexec_init_cpu_notes(const unsigned long cpu)
 {
-    Elf_Note * note = NULL;
+    struct elf_notes {
+        Elf_Note first;
+        unsigned char more[];
+    } *notes = NULL;
     int ret = 0;
     int nr_bytes = 0;
 
@@ -477,7 +480,8 @@ static int kexec_init_cpu_notes(const un
 
     /* If we dont care about the position of allocation, malloc. */
     if ( low_crashinfo_mode == LOW_CRASHINFO_NONE )
-        note = xzalloc_bytes(nr_bytes);
+        notes = xzalloc_flex_struct(struct elf_notes, more,
+                                    nr_bytes - sizeof(notes->first));
 
     /* Protect the write into crash_notes[] with a spinlock, as this function
      * is on a hotplug path and a hypercall path. */
@@ -490,26 +494,28 @@ static int kexec_init_cpu_notes(const un
         spin_unlock(&crash_notes_lock);
         /* Always return ok, because whether we successfully allocated or not,
          * another CPU has successfully allocated. */
-        xfree(note);
+        xfree(notes);
     }
     else
     {
         /* If we care about memory possition, alloc from the crash heap,
          * also protected by the crash_notes_lock. */
         if ( low_crashinfo_mode > LOW_CRASHINFO_NONE )
-            note = alloc_from_crash_heap(nr_bytes);
+            notes = alloc_from_crash_heap(nr_bytes);
 
-        crash_notes[cpu].start = note;
+        crash_notes[cpu].start = &notes->first;
         crash_notes[cpu].size = nr_bytes;
         spin_unlock(&crash_notes_lock);
 
         /* If the allocation failed, and another CPU did not beat us, give
          * up with ENOMEM. */
-        if ( ! note )
+        if ( ! notes )
             ret = -ENOMEM;
         /* else all is good so lets set up the notes. */
         else
         {
+            Elf_Note *note = &notes->first;
+
             /* Set up CORE note. */
             setup_note(note, "CORE", NT_PRSTATUS, sizeof(ELF_Prstatus));
             note = ELFNOTE_NEXT(note);