[PATCH 2/2] Add -mem-shared option

[Marc-André Lureau]

Add an option to simplify shared memory / vhost-user setup.

Currently, using vhost-user requires NUMA setup such as:
-m 4G -object memory-backend-file,id=mem,size=4G,mem-path=/dev/shm,share=on 
-numa node,memdev=mem

As there is no other way to allocate shareable RAM, afaik.

-mem-shared aims to have a simple way instead: -m 4G -mem-shared

Signed-off-by: Marc-André Lureau <address@hidden>
---
 exec.c                  | 11 ++++++++++-
 hw/core/numa.c          | 16 +++++++++++++++-
 include/sysemu/sysemu.h |  1 +
 qemu-options.hx         | 10 ++++++++++
 vl.c                    |  4 ++++
 5 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/exec.c b/exec.c
index ffdb518535..4e53937eaf 100644
--- a/exec.c
+++ b/exec.c
@@ -72,6 +72,10 @@
 #include "qemu/mmap-alloc.h"
 #endif
 
+#ifdef CONFIG_POSIX
+#include "qemu/memfd.h"
+#endif
+
 #include "monitor/monitor.h"
 
 //#define DEBUG_SUBPAGE
@@ -2347,7 +2351,12 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, 
MemoryRegion *mr,
     bool created;
     RAMBlock *block;
 
-    fd = file_ram_open(mem_path, memory_region_name(mr), &created, errp);
+    if (mem_path) {
+        fd = file_ram_open(mem_path, memory_region_name(mr), &created, errp);
+    } else {
+        fd = qemu_memfd_open(mr->name, size,
+                             F_SEAL_GROW | F_SEAL_SHRINK | F_SEAL_SEAL, errp);
+    }
     if (fd < 0) {
         return NULL;
     }
diff --git a/hw/core/numa.c b/hw/core/numa.c
index e3332a984f..6f72cddb1c 100644
--- a/hw/core/numa.c
+++ b/hw/core/numa.c
@@ -493,7 +493,8 @@ static void allocate_system_memory_nonnuma(MemoryRegion 
*mr, Object *owner,
     if (mem_path) {
 #ifdef __linux__
         Error *err = NULL;
-        memory_region_init_ram_from_file(mr, owner, name, ram_size, 0, 0,
+        memory_region_init_ram_from_file(mr, owner, name, ram_size, 0,
+                                         mem_shared ? RAM_SHARED : 0,
                                          mem_path, &err);
         if (err) {
             error_report_err(err);
@@ -513,6 +514,19 @@ static void allocate_system_memory_nonnuma(MemoryRegion 
*mr, Object *owner,
 #else
         fprintf(stderr, "-mem-path not supported on this host\n");
         exit(1);
+#endif
+    } else if (mem_shared) {
+#ifdef CONFIG_POSIX
+        Error *err = NULL;
+        memory_region_init_ram_from_file(mr, owner, NULL, ram_size, 0,
+                                         RAM_SHARED, NULL, &err);
+        if (err) {
+            error_report_err(err);
+            exit(1);
+        }
+#else
+        fprintf(stderr, "-mem-shared not supported on this host\n");
+        exit(1);
 #endif
     } else {
         memory_region_init_ram_nomigrate(mr, owner, name, ram_size, 
&error_fatal);
diff --git a/include/sysemu/sysemu.h b/include/sysemu/sysemu.h
index 80c57fdc4e..80db8465a9 100644
--- a/include/sysemu/sysemu.h
+++ b/include/sysemu/sysemu.h
@@ -55,6 +55,7 @@ extern bool enable_cpu_pm;
 extern QEMUClockType rtc_clock;
 extern const char *mem_path;
 extern int mem_prealloc;
+extern int mem_shared;
 
 #define MAX_OPTION_ROMS 16
 typedef struct QEMUOptionRom {
diff --git a/qemu-options.hx b/qemu-options.hx
index 65c9473b73..4c69b03ad3 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -394,6 +394,16 @@ STEXI
 Preallocate memory when using -mem-path.
 ETEXI
 
+DEF("mem-shared", 0, QEMU_OPTION_mem_shared,
+    "-mem-shared     allocate shared memory\n", QEMU_ARCH_ALL)
+STEXI
+@item -mem-shared
+@findex -mem-shared
+Allocate guest RAM with shared mapping.  Whether the allocation is
+anonymous or not (with -mem-path), QEMU will allocate a shared memory that
+can be shared by unrelated processes, such as vhost-user backends.
+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 6a65a64bfd..53b1155455 100644
--- a/vl.c
+++ b/vl.c
@@ -143,6 +143,7 @@ const char* keyboard_layout = NULL;
 ram_addr_t ram_size;
 const char *mem_path = NULL;
 int mem_prealloc = 0; /* force preallocation of physical target memory */
+int mem_shared = 0;
 bool enable_mlock = false;
 bool enable_cpu_pm = false;
 int nb_nics;
@@ -3172,6 +3173,9 @@ int main(int argc, char **argv, char **envp)
             case QEMU_OPTION_mem_prealloc:
                 mem_prealloc = 1;
                 break;
+            case QEMU_OPTION_mem_shared:
+                mem_shared = 1;
+                break;
             case QEMU_OPTION_d:
                 log_mask = optarg;
                 break;
-- 
2.24.0