s390/kdump: Provide is_kdump_kernel() implementation

s390 sets "elfcorehdr_addr = ELFCORE_ADDR_MAX;" early during
setup_arch() to deactivate the "elfcorehdr= kernel" parameter,
resulting in is_kdump_kernel() returning "false".

During vmcore_init()->elfcorehdr_alloc(), if on a dump kernel and
allocation succeeded, elfcorehdr_addr will be set to a valid address
and is_kdump_kernel() will consequently return "true".

is_kdump_kernel() should return a consistent result during all boot
stages, and properly return "true" if in a kdump environment - just
like it is done on powerpc where "false" is indicated in fadump
environments, as added in commit b098f1c323 ("powerpc/fadump: make
is_kdump_kernel() return false when fadump is active").

Similarly provide a custom is_kdump_kernel() implementation that will only
return "true" in kdump environments, and will do so consistently during
boot.

Update the documentation of dump_available().

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Link: https://lore.kernel.org/r/20241023090651.1115507-1-david@redhat.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/kexec.h

@@ -94,6 +94,9 @@ void arch_kexec_protect_crashkres(void);
 
 void arch_kexec_unprotect_crashkres(void);
 #define arch_kexec_unprotect_crashkres arch_kexec_unprotect_crashkres
+
+bool is_kdump_kernel(void);
+#define is_kdump_kernel is_kdump_kernel
 #endif
 
 #ifdef CONFIG_KEXEC_FILE

arch/s390/kernel/crash_dump.c

@@ -237,6 +237,17 @@ int remap_oldmem_pfn_range(struct vm_area_struct *vma, unsigned long from,
 						       prot);
 }
 
+/*
+ * Return true only when in a kdump or stand-alone kdump environment.
+ * Note that /proc/vmcore might also be available in "standard zfcp/nvme dump"
+ * environments, where this function returns false; see dump_available().
+ */
+bool is_kdump_kernel(void)
+{
+	return oldmem_data.start;
+}
+EXPORT_SYMBOL_GPL(is_kdump_kernel);
+
 static const char *nt_name(Elf64_Word type)
 {
 	const char *name = "LINUX";

arch/s390/kernel/smp.c

@@ -574,7 +574,7 @@ int smp_store_status(int cpu)
 
 /*
  * Collect CPU state of the previous, crashed system.
- * There are four cases:
+ * There are three cases:
  * 1) standard zfcp/nvme dump
  *    condition: OLDMEM_BASE == NULL && is_ipl_type_dump() == true
  *    The state for all CPUs except the boot CPU needs to be collected
@@ -587,16 +587,16 @@ int smp_store_status(int cpu)
  *    with sigp stop-and-store-status. The firmware or the boot-loader
  *    stored the registers of the boot CPU in the absolute lowcore in the
  *    memory of the old system.
- * 3) kdump and the old kernel did not store the CPU state,
- *    or stand-alone kdump for DASD
- *    condition: OLDMEM_BASE != NULL && !is_kdump_kernel()
+ * 3) kdump or stand-alone kdump for DASD
+ *    condition: OLDMEM_BASE != NULL && is_ipl_type_dump() == false
  *    The state for all CPUs except the boot CPU needs to be collected
  *    with sigp stop-and-store-status. The kexec code or the boot-loader
  *    stored the registers of the boot CPU in the memory of the old system.
- * 4) kdump and the old kernel stored the CPU state
- *    condition: OLDMEM_BASE != NULL && is_kdump_kernel()
- *    This case does not exist for s390 anymore, setup_arch explicitly
- *    deactivates the elfcorehdr= kernel parameter
+ *
+ * Note that the legacy kdump mode where the old kernel stored the CPU states
+ * does no longer exist: setup_arch() explicitly deactivates the elfcorehdr=
+ * kernel parameter. The is_kdump_kernel() implementation on s390 is independent
+ * of the elfcorehdr= parameter.
  */
 static bool dump_available(void)
 {