pc-bios: s390x: Clear out leftover S390EP string

A Linux binary will have the string "S390EP" at address 0x10008,
which is important in getting the guest up off the ground. In the
case of a reboot (specifically chreipl going to a new device),
we should defer to the PSW at address zero for the new config,
which will re-write "S390EP" from the new image.

Let's clear it out at this point so that a reipl to, say, a DASD
passthrough device drives the IPL path from scratch without disrupting
disrupting the order of operations for other boots.

Rather than hardcoding the address of this magic (again), let's
define it somewhere so that the two users are visibly related.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Message-Id: <20201120160117.59366-3-farman@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

pc-bios/s390-ccw/jump2ipl.c

@@ -78,7 +78,7 @@ void jump_to_low_kernel(void)
      * kernel start address (when jumping to the PSW-at-zero address instead,
      * the kernel startup code fails when we booted from a network device).
      */
-    if (!memcmp((char *)0x10008, "S390EP", 6)) {
+    if (!memcmp((char *)S390EP, "S390EP", 6)) {
         jump_to_IPL_code(KERN_IMAGE_START);
     }
 

pc-bios/s390-ccw/main.c

@@ -178,6 +178,12 @@ static void boot_setup(void)
     memcpy(lpmsg + 10, loadparm_str, 8);
     sclp_print(lpmsg);
 
+    /*
+     * Clear out any potential S390EP magic (see jump_to_low_kernel()),
+     * so we don't taint our decision-making process during a reboot.
+     */
+    memset((char *)S390EP, 0, 6);
+
     have_iplb = store_iplb(&iplb);
 }
 

pc-bios/s390-ccw/s390-arch.h

@@ -95,6 +95,9 @@ typedef struct LowCore {
 
 extern LowCore *lowcore;
 
+/* Location of "S390EP" in a Linux binary (see arch/s390/boot/head.S) */
+#define S390EP 0x10008
+
 static inline void set_prefix(uint32_t address)
 {
     asm volatile("spx %0" : : "m" (address) : "memory");