[Qemu-devel] [PATCH] Add option to mlock guest and qemu memory

[Satoru Moriya]

This is a first time for me to post a patch to qemu-devel.
If there is something missing/wrong, please let me know.

We have some plans to migrate old enterprise systems which require
low latency (msec order) to kvm virtualized environment. Usually,
we uses mlock to preallocate and pin down process memory in order
to avoid page allocation in latency critical path. On the other
hand, in kvm environment, mlocking in guests is not effective
because it can't avoid page reclaim in host. Actually, to avoid
guest memory reclaim, qemu has "mem-path" option that is actually
for using hugepage. But a memory region of qemu is not allocated
on hugepage, so it may be reclaimed. That may cause a latency
problem.

To avoid guest and qemu memory reclaim, this patch introduces
a new "mlock" option. With this option, we can preallocate and
pin down guest and qemu memory before booting guest OS.

Tested on Linux, x86_64 (fedora 17).

Signed-off-by: Satoru Moriya <address@hidden>
---
 cpu-all.h       | 1 +
 exec.c          | 3 +++
 qemu-options.hx | 8 ++++++++
 vl.c            | 4 ++++
 4 files changed, 16 insertions(+)

diff --git a/cpu-all.h b/cpu-all.h
index 74d3681..e12e5d5 100644
--- a/cpu-all.h
+++ b/cpu-all.h
@@ -503,6 +503,7 @@ extern RAMList ram_list;
 
 extern const char *mem_path;
 extern int mem_prealloc;
+extern int mem_lock;
 
 /* Flags stored in the low bits of the TLB virtual address.  These are
    defined so that fast path ram access is all zeros.  */
diff --git a/exec.c b/exec.c
index bb6aa4a..de13bc9 100644
--- a/exec.c
+++ b/exec.c
@@ -2572,6 +2572,9 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void 
*host,
             }
             memory_try_enable_merging(new_block->host, size);
         }
+        if (mem_lock && mlockall(MCL_CURRENT | MCL_FUTURE)) {
+            perror("mlockall");
+        }
     }
     new_block->length = size;
 
diff --git a/qemu-options.hx b/qemu-options.hx
index 7d97f96..9d82f15 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -427,6 +427,14 @@ Preallocate memory when using -mem-path.
 ETEXI
 #endif
 
+DEF("mlock", 0, QEMU_OPTION_mlock,
+    "-mlock          mlock guest and qemu memory\n",
+    QEMU_ARCH_ALL)
+STEXI
address@hidden -mlock
+mlock guest and qemu memory.
+ETEXI
+
 DEF("k", HAS_ARG, QEMU_OPTION_k,
     "-k language     use keyboard layout (for example 'fr' for French)\n",
     QEMU_ARCH_ALL)
diff --git a/vl.c b/vl.c
index 8d305ca..c902084 100644
--- a/vl.c
+++ b/vl.c
@@ -187,6 +187,7 @@ const char *mem_path = NULL;
 #ifdef MAP_POPULATE
 int mem_prealloc = 0; /* force preallocation of physical target memory */
 #endif
+int mem_lock;
 int nb_nics;
 NICInfo nd_table[MAX_NICS];
 int autostart;
@@ -2770,6 +2771,9 @@ int main(int argc, char **argv, char **envp)
                 mem_prealloc = 1;
                 break;
 #endif
+            case QEMU_OPTION_mlock:
+                mem_lock = 1;
+                break;
             case QEMU_OPTION_d:
                 log_mask = optarg;
                 break;
-- 
1.7.11.4