add -machine mem-merge=on|off option

It allows to disable memory merge support (KSM on Linux), which is
enabled by default otherwise.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

exec.c

@@ -2525,6 +2525,19 @@ void qemu_ram_set_idstr(ram_addr_t addr, const char *name, DeviceState *dev)
     }
 }
 
+static int memory_try_enable_merging(void *addr, size_t len)
+{
+    QemuOpts *opts;
+
+    opts = qemu_opts_find(qemu_find_opts("machine"), 0);
+    if (opts && !qemu_opt_get_bool(opts, "mem-merge", true)) {
+        /* disabled by the user */
+        return 0;
+    }
+
+    return qemu_madvise(addr, len, QEMU_MADV_MERGEABLE);
+}
+
 ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
                                    MemoryRegion *mr)
 {
@@ -2544,7 +2557,7 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
             new_block->host = file_ram_alloc(new_block, size, mem_path);
             if (!new_block->host) {
                 new_block->host = qemu_vmalloc(size);
-                qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE);
+                memory_try_enable_merging(new_block->host, size);
             }
 #else
             fprintf(stderr, "-mem-path option unsupported\n");
@@ -2559,7 +2572,7 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
             } else {
                 new_block->host = qemu_vmalloc(size);
             }
-            qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE);
+            memory_try_enable_merging(new_block->host, size);
         }
     }
     new_block->length = size;
@@ -2689,7 +2702,7 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
                             length, addr);
                     exit(1);
                 }
-                qemu_madvise(vaddr, length, QEMU_MADV_MERGEABLE);
+                memory_try_enable_merging(vaddr, length);
                 qemu_ram_setup_dump(vaddr, length);
             }
             return;

qemu-config.c

@@ -615,6 +615,10 @@ static QemuOptsList qemu_machine_opts = {
             .name = "dump-guest-core",
             .type = QEMU_OPT_BOOL,
             .help = "Include guest memory in  a core dump",
+        }, {
+            .name = "mem-merge",
+            .type = QEMU_OPT_BOOL,
+            .help = "enable/disable memory merge support",
         },
         { /* End of list */ }
     },

qemu-options.hx

@@ -38,7 +38,8 @@ DEF("machine", HAS_ARG, QEMU_OPTION_machine, \
     "                supported accelerators are kvm, xen, tcg (default: tcg)\n"
     "                kernel_irqchip=on|off controls accelerated irqchip support\n"
     "                kvm_shadow_mem=size of KVM shadow MMU\n"
-    "                dump-guest-core=on|off include guest memory in a core dump (default=on)\n",
+    "                dump-guest-core=on|off include guest memory in a core dump (default=on)\n"
+    "                mem-merge=on|off controls memory merge support (default: on)\n",
     QEMU_ARCH_ALL)
 STEXI
 @item -machine [type=]@var{name}[,prop=@var{value}[,...]]
@@ -57,6 +58,10 @@ Enables in-kernel irqchip support for the chosen accelerator when available.
 Defines the size of the KVM shadow MMU.
 @item dump-guest-core=on|off
 Include guest memory in a core dump. The default is on.
+@item mem-merge=on|off
+Enables or disables memory merge support. This feature, when supported by
+the host, de-duplicates identical memory pages among VMs instances
+(enabled by default).
 @end table
 ETEXI