s390: avoid cache aliasing under z/VM and KVM

commit 1f6b83e5e4 ("s390: avoid z13 cache aliasing") checks for the
machine type to optimize address space randomization and zero page
allocation to avoid cache aliases.

This check might fail under a hypervisor with migration support.
z/VMs "Single System Image and Live Guest Relocation" facility will
"fake" the machine type of the oldest system in the group. For example
in a group of zEC12 and Z13 the guest appears to run on a zEC12
(architecture fencing within the relocation domain)

Remove the machine type detection and always use cache aliasing
rules that are known to work for all machines. These are the z13
aliasing rules.

Suggested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/process.c

@@ -243,11 +243,7 @@ unsigned long arch_align_stack(unsigned long sp)
 
 static inline unsigned long brk_rnd(void)
 {
-	/* 8MB for 32bit, 1GB for 64bit */
-	if (is_32bit_task())
-		return (get_random_int() & 0x7ffUL) << PAGE_SHIFT;
-	else
-		return (get_random_int() & 0x3ffffUL) << PAGE_SHIFT;
+	return (get_random_int() & BRK_RND_MASK) << PAGE_SHIFT;
 }
 
 unsigned long arch_randomize_brk(struct mm_struct *mm)