[Qemu-devel] [PATCH RFC 4/6] vhost: alloc shareable log

["Marc-André Lureau" <marcandre.lureau@redhat.com>]

If the backend is of type VHOST_BACKEND_TYPE_USER, allocate
shareable memory.

Note: vhost_log_get() can use a global "vhost_log" that can be shared by
several vhost devices. We may want instead a common shareable log and a
common non-shareable one.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 hw/virtio/vhost.c         | 42 +++++++++++++++++++++++++++++++++---------
 include/hw/virtio/vhost.h |  3 ++-
 2 files changed, 35 insertions(+), 10 deletions(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 2712c6f..12dd644 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -286,20 +286,34 @@ static uint64_t vhost_get_log_size(struct vhost_dev *dev)
     }
     return log_size;
 }
-static struct vhost_log *vhost_log_alloc(uint64_t size)
+
+static struct vhost_log *vhost_log_alloc(uint64_t size, bool share)
 {
-    struct vhost_log *log = g_malloc0(sizeof *log + size * sizeof(*(log->log)));
+    struct vhost_log *log;
+    uint64_t logsize = size * sizeof(*(log->log));
+    int fd = -1;
+
+    log = g_new0(struct vhost_log, 1);
+    if (share) {
+        log->log = qemu_memfd_alloc("vhost-log", logsize,
+                                    F_SEAL_GROW|F_SEAL_SHRINK|F_SEAL_SEAL, &fd);
+        memset(log->log, 0, logsize);
+    } else {
+        log->log = g_malloc0(logsize);
+    }
 
     log->size = size;
     log->refcnt = 1;
+    log->fd = fd;
 
     return log;
 }
 
-static struct vhost_log *vhost_log_get(uint64_t size)
+static struct vhost_log *vhost_log_get(uint64_t size, bool share)
 {
-    if (!vhost_log || vhost_log->size != size) {
-        vhost_log = vhost_log_alloc(size);
+    if (!vhost_log || vhost_log->size != size ||
+        (share && vhost_log->fd == -1)) {
+        vhost_log = vhost_log_alloc(size, share);
     } else {
         ++vhost_log->refcnt;
     }
@@ -324,21 +338,30 @@ static void vhost_log_put(struct vhost_dev *dev, bool sync)
         if (vhost_log == log) {
             vhost_log = NULL;
         }
+
+        if (log->fd == -1) {
+            g_free(log->log);
+        } else {
+            qemu_memfd_free(log->log, log->size * sizeof(*(log->log)),
+                            log->fd);
+        }
         g_free(log);
     }
 }
 
 static inline void vhost_dev_log_resize(struct vhost_dev* dev, uint64_t size)
 {
-    struct vhost_log *log = vhost_log_get(size);
+    bool share = dev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER;
+    struct vhost_log *log = vhost_log_get(size, share);
     uint64_t log_base = (uintptr_t)log->log;
     int r;
 
-    r = dev->vhost_ops->vhost_call(dev, VHOST_SET_LOG_BASE, &log_base);
-    assert(r >= 0);
     vhost_log_put(dev, true);
     dev->log = log;
     dev->log_size = size;
+
+    r = dev->vhost_ops->vhost_call(dev, VHOST_SET_LOG_BASE, &log_base);
+    assert(r >= 0);
 }
 
 static int vhost_verify_ring_mappings(struct vhost_dev *dev,
@@ -1136,9 +1159,10 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
 
     if (hdev->log_enabled) {
         uint64_t log_base;
+        bool share = hdev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER;
 
         hdev->log_size = vhost_get_log_size(hdev);
-        hdev->log = vhost_log_get(hdev->log_size);
+        hdev->log = vhost_log_get(hdev->log_size, share);
         log_base = (uintptr_t)hdev->log->log;
         r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_LOG_BASE,
                                         hdev->log_size ? &log_base : NULL);
diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index 6467c73..ab1dcac 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -31,7 +31,8 @@ typedef unsigned long vhost_log_chunk_t;
 struct vhost_log {
     unsigned long long size;
     int refcnt;
-    vhost_log_chunk_t log[0];
+    int fd;
+    vhost_log_chunk_t *log;
 };
 
 struct vhost_memory;
-- 
2.4.3