virtio: add the vhost-vsock backend for virtio-vsock by will-j-wright · Pull Request #4006 · microsoft/openvmm

will-j-wright · 2026-07-22T22:08:47Z

Add the vhost-vsock backend for virtio-vsock, implemented as VhostVsockDevice. This backend manifests as AF_VSOCK on both ends of the connection by delegating the virtqueues to the kernel with /dev/vhost-vsock, rather than the user-mode AF_UNIX/AF_VSOCK hybrid model used by VirtioVsockDevice. This exposes the guest in the host's AF_VSOCK namespace, allowing users on the host to connect(AF_VSOCK, CID, port) to reach services inside guest.

Also add a new petri test exercising this scenario.

github-actions · 2026-07-22T22:09:08Z

⚠️ Unsafe Code Detected

This PR modifies files containing unsafe Rust code. Extra scrutiny is required during review.

For more on why we check whole files, instead of just diffs, check out the Rustonomicon

Copilot

Pull request overview

Adds a Linux-only virtio-vsock backend that delegates the virtqueue data path to the kernel via /dev/vhost-vsock, making the guest reachable from the host’s AF_VSOCK namespace. This integrates the backend end-to-end (resource handle + resolver + CLI + docs) and extends Petri/CI to exercise it.

Changes:

Introduce VhostVsockDevice (Linux vhost_vsock) implementation in virtio_vsock.
Wire the backend through virtio_resources, resolver registration, and a new openvmm CLI flag --virtio-vsock-vhost-cid.
Update Petri to connect to pipette over host AF_VSOCK, add a new Petri boot test, and prepare /dev/vhost-vsock in the nextest/flowey job.

Reviewed changes

Copilot reviewed 17 out of 18 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
vmm_tests/vmm_tests/tests/tests/multiarch.rs	Adds a Linux boot test that uses the vhost-vsock backend and validates host-to-guest AF_VSOCK connectivity via pipette.
vm/devices/virtio/virtio_vsock/src/vhost.rs	New Linux vhost-vsock virtio-vsock device implementation + unit tests for CID validation, queue state encoding, and memory mapping.
vm/devices/virtio/virtio_vsock/src/resolver.rs	Adds a Linux-only static resolver for the new vhost-vsock handle.
vm/devices/virtio/virtio_vsock/src/lib.rs	Plumbs in the Linux-only `vhost` module and updates the crate-level unsafe rationale comment.
vm/devices/virtio/virtio_vsock/Cargo.toml	Adds Linux-only dependencies needed by the vhost backend (`nix` ioctl, `pal_event`, `sparse_mmap`).
vm/devices/virtio/virtio_resources/src/lib.rs	Adds `VirtioVsockVhostHandle` MeshPayload resource ID for selecting the vhost backend.
petri/src/vm/openvmm/runtime.rs	Adds AF_VSOCK-based pipette connection path when a vhost-vsock CID is configured.
petri/src/vm/openvmm/construct.rs	Chooses between user-mode vsock vs vhost-vsock resources and enforces shared-memory requirements when vhost-vsock is enabled.
petri/src/vm/mod.rs	Extends `PetriVmBuilder`/properties with a Linux-only vhost-vsock guest CID knob (`with_vhost_vsock`).
petri/Cargo.toml	Adds Linux-only `vmsocket` dependency for AF_VSOCK connections.
openvmm/openvmm_resources/src/lib.rs	Registers the new Linux-only virtio-vsock vhost resolver.
openvmm/openvmm_entry/src/lib.rs	Adds device creation from the new CLI option (`--virtio-vsock-vhost-cid`).
openvmm/openvmm_entry/src/cli_args.rs	Defines CLI flag, CID parser/validation, and tests for the new option.
Guide/src/reference/openvmm/management/cli.md	Documents `--virtio-vsock-path` and new `--virtio-vsock-vhost-cid` behavior/requirements.
flowey/flowey_lib_hvlite/src/test_nextest_vmm_tests_archive.rs	Adds an optional pre-test step to `modprobe vhost_vsock` and chmod `/dev/vhost-vsock`.
flowey/flowey_lib_hvlite/src/_jobs/local_build_and_run_nextest_vmm_tests.rs	Threads through the new `prepare_vhost_vsock` option (disabled for this local job).
flowey/flowey_lib_hvlite/src/_jobs/consume_and_test_nextest_vmm_tests_archive.rs	Enables vhost-vsock preparation for Linux x86_64 nextest runs (where the new test applies).
Cargo.lock	Records new dependency usage (notably `nix` and `sparse_mmap`).

Copilot

Pull request overview

Copilot reviewed 17 out of 18 changed files in this pull request and generated 1 comment.

chris-oo

i didn't look too much at vhost.rs, i need a bit more context to really review it / john perhaps has the right context

chris-oo · 2026-07-22T22:27:34Z

            //   that requires shared, file-backed memory.
-            let private_incompatible = firmware.is_openhcl() || firmware.is_pcat();
+            let private_incompatible =
+                firmware.is_openhcl() || firmware.is_pcat() || vhost_vsock_guest_cid.is_some();


this needs a comment why this is private incompatible (because the kernel needs access, or why?)

does the bail below not have enough detail?

Copilot

Pull request overview

Copilot reviewed 17 out of 18 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

vm/devices/virtio/virtio_vsock/src/vhost.rs:17

size_of is used unqualified in this module (e.g. when building the vhost memory table and device register length), but it isn’t imported into scope anywhere in the file. This will fail to compile unless each call is qualified (e.g. std::mem::size_of). Add a use core::mem::size_of; (or std::mem::size_of) near the other imports.

use crate::spec::VsockConfig;
use anyhow::Context as _;
use guestmem::GuestMemory;
use inspect::InspectMut;
use pal_event::Event;

Copilot

Pull request overview

Copilot reviewed 21 out of 22 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

vm/devices/virtio/virtio_vsock/src/vhost.rs:22

size_of::<...>() is used in this module (e.g. when building the vhost memory table), but size_of isn't imported, so this file won't compile on Linux. Import size_of from std::mem (or core::mem).

use pal_event::Event;
use sparse_mmap::SparseMapping;
use std::os::fd::AsFd as _;
use std::os::fd::AsRawFd as _;
use std::os::fd::OwnedFd;
use std::os::fd::RawFd;

github-actions · 2026-07-22T23:56:14Z

1 Petri tests failed (1 unstable)

Copilot

Pull request overview

Copilot reviewed 21 out of 22 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

vm/devices/virtio/virtio_vsock/src/vhost.rs:17

This module calls size_of::<...>() (e.g. when building the vhost memory table), but size_of isn’t imported, so the file won’t compile as-is. Add an explicit use std::mem::size_of; near the other imports.

use crate::spec::VsockConfig;
use anyhow::Context as _;
use guestmem::GuestMemory;
use inspect::InspectMut;
use pal_event::Event;

+                    match self.memory().read_used_index(&runtime.params) {
+                        Ok(index) => index,
+                        Err(error) => {
+                            tracelimit::warn_ratelimited!(
+                                error = &*error as &dyn std::error::Error,
+                                index,
+                                "failed to read vhost-vsock used index"
+                            );
+                            return None;
+                        }
+                    }


will-j-wright assigned chris-oo Jul 22, 2026

Copilot AI review requested due to automatic review settings July 22, 2026 22:08

will-j-wright requested a review from a team as a code owner July 22, 2026 22:08

github-actions Bot added the Guide label Jul 22, 2026

github-actions Bot added the unsafe Related to unsafe code label Jul 22, 2026

Copilot started reviewing on behalf of will-j-wright July 22, 2026 22:09 View session

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Comment thread Guide/src/reference/openvmm/management/cli.md Outdated

Comment thread vm/devices/virtio/virtio_vsock/src/vhost.rs Outdated

Copilot AI review requested due to automatic review settings July 22, 2026 22:20

will-j-wright force-pushed the vhost-vsock branch from 71e831d to ad62631 Compare July 22, 2026 22:20

chris-oo assigned jstarks Jul 22, 2026

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Comment thread vm/devices/virtio/virtio_vsock/src/vhost.rs

Copilot AI review requested due to automatic review settings July 22, 2026 22:29

will-j-wright force-pushed the vhost-vsock branch from ad62631 to 6cec5b8 Compare July 22, 2026 22:29

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will-j-wright force-pushed the vhost-vsock branch from 6cec5b8 to 4738e22 Compare July 22, 2026 22:42

Copilot AI review requested due to automatic review settings July 22, 2026 22:42

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jstarks reviewed Jul 23, 2026

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Comment thread vm/devices/virtio/virtio_vsock/src/vhost.rs

jstarks reviewed Jul 23, 2026

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Comment thread vm/devices/virtio/virtio_vsock/src/vhost.rs

jstarks reviewed Jul 23, 2026

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Comment thread vm/devices/virtio/virtio_resources/src/lib.rs

will-j-wright added 2 commits July 24, 2026 17:36

virtio_vsock: add kernel vhost_vsock backend

c2178a9

virtio_vsock: add kernel backend VMM test

8201d63

will-j-wright force-pushed the vhost-vsock branch from 4738e22 to 8201d63 Compare July 24, 2026 17:48

Copilot AI review requested due to automatic review settings July 24, 2026 17:48

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