Allocate physical memory in low virtual address space

KVM on S390x requires the virtual address space of the guest's RAM to be
within the first 256GB.

The general direction I'd like to see KVM on S390 move is that this requirement
is losened, but for now that's what we're stuck with.

So let's just hack up qemu_ram_alloc until KVM behaves nicely :-).
Signed-off-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

exec.c

@@ -2411,7 +2411,13 @@ ram_addr_t qemu_ram_alloc(ram_addr_t size)
     size = TARGET_PAGE_ALIGN(size);
     new_block = qemu_malloc(sizeof(*new_block));
 
+#if defined(TARGET_S390X) && defined(CONFIG_KVM)
+    /* XXX S390 KVM requires the topmost vma of the RAM to be < 256GB */
+    new_block->host = mmap((void*)0x1000000, size, PROT_EXEC|PROT_READ|PROT_WRITE,
+                           MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+#else
     new_block->host = qemu_vmalloc(size);
+#endif
 #ifdef MADV_MERGEABLE
     madvise(new_block->host, size, MADV_MERGEABLE);
 #endif