[Qemu-devel] [PATCH v2] add -machine mem_merge=on|off option

[Luiz Capitulino]

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

Signed-off-by: Luiz Capitulino <address@hidden>
---

o v2: improve manpage description

 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 8244d54..24dd154 100644
--- a/exec.c
+++ b/exec.c
@@ -2499,6 +2499,19 @@ void qemu_ram_set_idstr(ram_addr_t addr, const char 
*name, DeviceState *dev)
     }
 }
 
+static int madvise_mergeable(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)
 {
@@ -2518,7 +2531,7 @@ 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);
+                madvise_mergeable(new_block->host, size);
             }
 #else
             fprintf(stderr, "-mem-path option unsupported\n");
@@ -2545,7 +2558,7 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void 
*host,
                 new_block->host = qemu_vmalloc(size);
             }
 #endif
-            qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE);
+            madvise_mergeable(new_block->host, size);
         }
     }
     new_block->length = size;
@@ -2672,7 +2685,7 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
                             length, addr);
                     exit(1);
                 }
-                qemu_madvise(vaddr, length, QEMU_MADV_MERGEABLE);
+                madvise_mergeable(vaddr, length);
             }
             return;
         }
diff --git a/qemu-config.c b/qemu-config.c
index 5c3296b..e47612e 100644
--- a/qemu-config.c
+++ b/qemu-config.c
@@ -595,6 +595,10 @@ static QemuOptsList qemu_machine_opts = {
             .name = "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 8b66264..15351a5 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 address@hidden,address@hidden,...]]
@@ -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.
address@hidden 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
 
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