Re: [PATCH v2] vhost-vdpa: allow passing opened vhostfd to vhost-vdpa

[Si-Wei Liu]

On 10/8/2022 10:43 PM, Jason Wang wrote:
On Sat, Oct 8, 2022 at 5:04 PM Si-Wei Liu<si-wei....@oracle.com>  wrote:
Similar to other vhost backends, vhostfd can be passed to vhost-vdpa
backend as another parameter to instantiate vhost-vdpa net client.
This would benefit the use case where only open file descriptors, as
opposed to raw vhost-vdpa device paths, are accessible from the QEMU
process.

(qemu) netdev_add type=vhost-vdpa,vhostfd=61,id=vhost-vdpa1
Adding Cindy.

This has been discussed before, we've already had
vhostdev=/dev/fdset/$fd which should be functional equivalent to what
has been proposed here. (And this is how libvirt works if I understand
correctly).
Yes, I was aware of that discussion. However, our implementation of the 
management software is a bit different from libvirt, in which the paths 
in /dev/fdset/NNN can't be dynamically passed to the container where 
QEMU is running. By using a specific vhostfd property with existing 
code, it would allow our mgmt software smooth adaption without having to 
add too much infra code to support the /dev/fdset/NNN trick.

On the other hand, the other vhost backends, e.g. tap (via vhost-net), 
vhost-scsi and vhost-vsock all accept vhostfd as parameter to 
instantiate device, although the /dev/fdset trick also works there. I 
think vhost-vdpa is not  unprecedented in this case?

Thanks,
-Siwei



Thanks

Signed-off-by: Si-Wei Liu<si-wei....@oracle.com>
Acked-by: Eugenio Pérez<epere...@redhat.com>

---
v2:
   - fixed typo in commit message
   - s/fd's/file descriptors/
---
  net/vhost-vdpa.c | 25 ++++++++++++++++++++-----
  qapi/net.json    |  3 +++
  qemu-options.hx  |  6 ++++--
  3 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/net/vhost-vdpa.c b/net/vhost-vdpa.c
index 182b3a1..366b070 100644
--- a/net/vhost-vdpa.c
+++ b/net/vhost-vdpa.c
@@ -683,14 +683,29 @@ int net_init_vhost_vdpa(const Netdev *netdev, const char 
*name,

      assert(netdev->type == NET_CLIENT_DRIVER_VHOST_VDPA);
      opts = &netdev->u.vhost_vdpa;
-    if (!opts->vhostdev) {
-        error_setg(errp, "vdpa character device not specified with vhostdev");
+    if (!opts->has_vhostdev && !opts->has_vhostfd) {
+        error_setg(errp,
+                   "vhost-vdpa: neither vhostdev= nor vhostfd= was specified");
          return -1;
      }

-    vdpa_device_fd = qemu_open(opts->vhostdev, O_RDWR, errp);
-    if (vdpa_device_fd == -1) {
-        return -errno;
+    if (opts->has_vhostdev && opts->has_vhostfd) {
+        error_setg(errp,
+                   "vhost-vdpa: vhostdev= and vhostfd= are mutually 
exclusive");
+        return -1;
+    }
+
+    if (opts->has_vhostdev) {
+        vdpa_device_fd = qemu_open(opts->vhostdev, O_RDWR, errp);
+        if (vdpa_device_fd == -1) {
+            return -errno;
+        }
+    } else if (opts->has_vhostfd) {
+        vdpa_device_fd = monitor_fd_param(monitor_cur(), opts->vhostfd, errp);
+        if (vdpa_device_fd == -1) {
+            error_prepend(errp, "vhost-vdpa: unable to parse vhostfd: ");
+            return -1;
+        }
      }

      r = vhost_vdpa_get_features(vdpa_device_fd, &features, errp);
diff --git a/qapi/net.json b/qapi/net.json
index dd088c0..926ecc8 100644
--- a/qapi/net.json
+++ b/qapi/net.json
@@ -442,6 +442,8 @@
  # @vhostdev: path of vhost-vdpa device
  #            (default:'/dev/vhost-vdpa-0')
  #
+# @vhostfd: file descriptor of an already opened vhost vdpa device
+#
  # @queues: number of queues to be created for multiqueue vhost-vdpa
  #          (default: 1)
  #
@@ -456,6 +458,7 @@
  { 'struct': 'NetdevVhostVDPAOptions',
    'data': {
      '*vhostdev':     'str',
+    '*vhostfd':      'str',
      '*queues':       'int',
      '*x-svq':        {'type': 'bool', 'features' : [ 'unstable'] } } }

diff --git a/qemu-options.hx b/qemu-options.hx
index 913c71e..c040f74 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -2774,8 +2774,10 @@ DEF("netdev", HAS_ARG, QEMU_OPTION_netdev,
      "                configure a vhost-user network, backed by a chardev 
'dev'\n"
  #endif
  #ifdef __linux__
-    "-netdev vhost-vdpa,id=str,vhostdev=/path/to/dev\n"
+    "-netdev vhost-vdpa,id=str[,vhostdev=/path/to/dev][,vhostfd=h]\n"
      "                configure a vhost-vdpa network,Establish a vhost-vdpa 
netdev\n"
+    "                use 'vhostdev=/path/to/dev' to open a vhost vdpa device\n"
+    "                use 'vhostfd=h' to connect to an already opened vhost vdpa 
device\n"
  #endif
  #ifdef CONFIG_VMNET
      "-netdev vmnet-host,id=str[,isolated=on|off][,net-uuid=uuid]\n"
@@ -3280,7 +3282,7 @@ SRST
               -netdev type=vhost-user,id=net0,chardev=chr0 \
               -device virtio-net-pci,netdev=net0

-``-netdev vhost-vdpa,vhostdev=/path/to/dev``
+``-netdev vhost-vdpa[,vhostdev=/path/to/dev][,vhostfd=h]``
      Establish a vhost-vdpa netdev.

      vDPA device is a device that uses a datapath which complies with
--
1.8.3.1