s390: autodetect map private

By default qemu will use MAP_PRIVATE for guest pages. This will write
protect pages and thus break on s390 systems that dont support this feature.
Therefore qemu has a hack to always use MAP_SHARED for s390. But MAP_SHARED
has other problems (no dirty pages tracking, a lot more swap overhead etc.)
Newer systems allow the distinction via KVM_CAP_S390_COW. With this feature
qemu can use the standard qemu alloc if available, otherwise it will use
the old s390 hack.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Alexander Graf <agraf@suse.de>

1 files changed, 3 insertions, 15 deletions

diff --git a/exec.c b/exec.c
index dd4833d..c9fa17d 100644
--- a/exec.c
+++ b/exec.c
@@ -2536,26 +2536,14 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
             exit(1);
 #endif
         } else {
-#if defined(TARGET_S390X) && defined(CONFIG_KVM)
-            /* S390 KVM requires the topmost vma of the RAM to be smaller than
-               an system defined value, which is at least 256GB. Larger systems
-               have larger values. We put the guest between the end of data
-               segment (system break) and this value. We use 32GB as a base to
-               have enough room for the system break to grow. */
-            new_block->host = mmap((void*)0x800000000, size,
-                                   PROT_EXEC|PROT_READ|PROT_WRITE,
-                                   MAP_SHARED | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
-            if (new_block->host == MAP_FAILED) {
-                fprintf(stderr, "Allocating RAM failed\n");
-                abort();
-            }
-#else
             if (xen_enabled()) {
                 xen_ram_alloc(new_block->offset, size, mr);
+            } else if (kvm_enabled()) {
+                /* some s390/kvm configurations have special constraints */
+                new_block->host = kvm_vmalloc(size);
             } else {
                 new_block->host = qemu_vmalloc(size);
             }
-#endif
             qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE);
         }
     }