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

[Jason Wang]

在 2022/10/12 13:59, Si-Wei Liu 写道:


On 10/11/2022 8:09 PM, Jason Wang wrote:
On Tue, Oct 11, 2022 at 1:18 AM Si-Wei Liu<si-wei....@oracle.com>  wrote:
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.
I think fdset has extra flexibility in e.g hot-plug to allow the file
descriptor to be passed with SCM_RIGHTS.
Yes, that's exactly the use case we'd like to support. Though the 
difference in our mgmt software stack from libvirt is that any dynamic 
path in /dev (like /dev/fdset/ABC or /dev/vhost-vdpa-XYZ) can't be 
allowed to get passed through to the container running QEMU on the fly 
for security reasons. fd passing is allowed, though, with very strict 
security checks.


Interesting, any reason for disallowing fd passing? I'm asking since 
it's the way that libvirt work and it seems to me we don't get any 
complaints in the past.


That's the main motivation for this direct vhostfd passing support 
(noted fdset doesn't need to be used along with /dev/fdset node).

Having it said, I found there's also nuance in the 
vhostdev=/dev/fdset/XyZ interface besides the /dev node limitation: 
the fd to open has to be dup'ed from the original one passed via 
SCM_RIGHTS. This also has implication on security that any ioctl call 
from QEMU can't be audited through the original fd.


I'm not sure I get this, but management layer can enforce a ioctl 
whiltelist for safety.

Thanks


With this regard, I think vhostfd offers more flexibility than work 
around those qemu_open() specifics. Would these justify the use case 
of concern?

Thanks,
-Siwei

  It would still be good to add
the support.

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?
Yes.

Thanks

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