[Qemu-devel] [PATCH v4] add -machine mem-merge=on|off option

[Qemu-devel] [PATCH v4] add -machine mem-merge=on|off option

[Luiz Capitulino]

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

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---

IMPORTANT: this is on top of this series:

   http://lists.gnu.org/archive/html/qemu-devel/2012-07/msg01798.html

o v4

- rename option to mem-merge
- rebase on top of latest master
- rebase on top of the machine option rename series

 exec.c          | 19 ++++++++++++++++---
 qemu-config.c   |  4 ++++
 qemu-options.hx |  7 ++++++-
 3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/exec.c b/exec.c
index c9fa17d..59d29ae 100644
--- a/exec.c
+++ b/exec.c
@@ -2510,6 +2510,19 @@ 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)
 {
@@ -2529,7 +2542,7 @@ 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");
@@ -2544,7 +2557,7 @@ 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;
@@ -2670,7 +2683,7 @@ 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);
             }
             return;
         }
diff --git a/qemu-config.c b/qemu-config.c
index 9dac3be..3355e45 100644
--- a/qemu-config.c
+++ b/qemu-config.c
@@ -599,6 +599,10 @@ static QemuOptsList qemu_machine_opts = {
             .alias= "dt_compatible",
             .type = QEMU_OPT_STRING,
             .help = "Overrides the \"compatible\" property of the dt root node",
+        }, {
+            .name = "mem-merge",
+            .type = QEMU_OPT_BOOL,
+            .help = "enable/disable memory merge support",
         },
         { /* End of list */ }
     },
diff --git a/qemu-options.hx b/qemu-options.hx
index 90eab87..25ce07e 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -33,7 +33,8 @@ DEF("machine", HAS_ARG, QEMU_OPTION_machine, \
     "                property accel=accel1[:accel2[:...]] selects accelerator\n"
     "                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",
+    "                kvm-shadow-mem=size of KVM shadow MMU\n"
+    "                mem-merge=on|off controls memory merge support (default: on)\n",
     QEMU_ARCH_ALL)
 STEXI
 @item -machine [type=]@var{name}[,prop=@var{value}[,...]]
@@ -50,6 +51,10 @@ to initialize.
 Enables in-kernel irqchip support for the chosen accelerator when available.
 @item kvm-shadow-mem=size
 Defines the size of the KVM shadow MMU.
+@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
 
-- 
1.7.11.1.116.g8228a23