virtio_net: enable checksum and GSO offloads by jstarks · Pull Request #2968 · microsoft/openvmm

jstarks · 2026-03-13T00:30:20Z

Add TX and RX offload support to the virtio-net device.

TX path:

Query the endpoint's TxOffloadSupport at device creation and advertise the corresponding virtio features (VIRTIO_NET_F_CSUM, GUEST_CSUM, HOST_TSO4, HOST_TSO6).
Parse the virtio-net header on each TX packet to extract checksum and GSO fields. Translate VIRTIO_NET_HDR_F_NEEDS_CSUM into TxFlags (offload_tcp_checksum / offload_udp_checksum / offload_ip_header_checksum) by inspecting csum_offset (16 = TCP, 6 = UDP) and determining IP version from the GSO type or the Ethernet header's EtherType.
For TSO packets (gso_type TCPV4/TCPV6), populate max_tcp_segment_size, l2/l3/l4 header lengths, and the segmentation offload flag.
Peek at the first 18 bytes of the Ethernet frame (enough for a VLAN tag) to derive l2_len and IP version when the GSO type alone is insufficient.

RX path:

Set VIRTIO_NET_HDR_F_DATA_VALID in the virtio-net header when both the IP and L4 checksums are reported as valid by the backend, allowing the guest to skip re-verification.

Hardening:

Gate offload parsing on negotiated features: only honor NEEDS_CSUM when VIRTIO_NET_F_CSUM was negotiated, only honor GSO when HOST_TSO4/HOST_TSO6 was negotiated. Store negotiated features in the Worker.
Validate header lengths before enabling offloads: require l2_len > 0, csum_start > l2_len, csum_start <= packet_len for checksum offloads. Require gso_size > 0, l3_len > 0, l4_len > 0 for TSO.
Make packet errors non-fatal: malformed guest descriptors (empty, too small, too many segments, unreadable) are completed with 0 bytes, logged rate-limited, and counted via a new tx_dropped counter — the worker continues processing.
Split TX packet queuing into try_queue_tx_packet (returns Result) and queue_tx_packet (handles cleanup structurally: truncate segments, complete descriptor, increment counter).
Avoid per-packet Vec allocation by pushing segments directly into the shared tx_segments buffer, with truncate-on-error rollback.

Tests:

Unit tests for parse_tx_offloads covering no-offloads, TCP/UDP checksum, IPv4/IPv6, TSO4/TSO6, TSO-without-needs_csum, and VLAN-tagged frames.
End-to-end TX test posting a packet with checksum offload flags through the full virtio queue path, asserting on TxMetadata flags, header lengths, and packet length.
RX tests verifying data_valid is set/cleared for Good, Unknown, Bad, and ValidatedButWrong checksum states.
Feature negotiation tests confirming correct VIRTIO_NET_F_* advertisement based on endpoint capabilities.

Copilot

Pull request overview

This PR adds virtio-net TX/RX offload support by advertising checksum/TSO features based on backend capabilities, translating virtio-net header offload fields into net_backend::TxMetadata, and setting VIRTIO_NET_HDR_F_DATA_VALID on RX when the backend reports validated checksums.

Changes:

Advertise VIRTIO_NET_F_CSUM and VIRTIO_NET_F_HOST_TSO4/6 based on Endpoint::tx_offload_support().
Parse virtio-net TX header + Ethernet prefix to set TxMetadata offload flags and header-length fields.
Set RX data_valid flag when backend checksum states are validated; add/extend unit and integration-style tests.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
vm/devices/virtio/virtio_net/src/lib.rs	Adds feature advertisement and TX offload parsing to populate `TxMetadata` for the backend.
vm/devices/virtio/virtio_net/src/buffers.rs	Sets RX virtio-net header `DATA_VALID` flag based on backend checksum validity.
vm/devices/virtio/virtio_net/src/tests.rs	Adds TX offload parsing tests, RX `data_valid` tests, and feature negotiation tests.

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Copilot

Pull request overview

Adds virtio-net TX/RX offload plumbing so the device can advertise and translate virtio-net checksum/GSO fields into backend TxMetadata/RX header flags, with accompanying tests.

Changes:

Advertise virtio-net offload-related feature bits based on backend TxOffloadSupport.
Parse virtio-net TX header fields (csum/GSO) and map them into net_backend::TxMetadata offload flags and header lengths.
Set VIRTIO_NET_HDR_F_DATA_VALID on RX when backend checksum state is reported as valid; add unit/integration tests for offload behaviors.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 10 comments.

File	Description
vm/devices/virtio/virtio_net/src/lib.rs	Offload feature advertisement + TX header peeking/parsing into `TxMetadata`.
vm/devices/virtio/virtio_net/src/buffers.rs	Map RX checksum validity into virtio-net header `data_valid` flag.
vm/devices/virtio/virtio_net/src/tests.rs	Add TX offload parsing tests, RX `data_valid` tests, and feature negotiation tests.

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Copilot

Pull request overview

Adds virtio-net TX/RX offload plumbing (checksum + TCP segmentation) and corresponding test coverage, integrating backend offload capabilities into virtio feature advertisement and per-packet metadata.

Changes:

Advertise virtio-net checksum/TSO features based on Endpoint::tx_offload_support() and propagate parsed offload metadata into TxMetadata.
Parse virtio-net TX headers + Ethernet EtherType/VLAN prefix to derive TxFlags and header lengths for checksum/GSO offloads.
Set RX VIRTIO_NET_HDR_F_DATA_VALID based on backend-reported checksum validity, with new unit/integration tests.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 7 comments.

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File	Description
vm/devices/virtio/virtio_net/src/lib.rs	Feature negotiation + TX header parsing to generate backend offload metadata.
vm/devices/virtio/virtio_net/src/buffers.rs	Map RX checksum validation state to virtio-net header `data_valid` flag.
vm/devices/virtio/virtio_net/src/tests.rs	Adds parsing/unit tests, RX `data_valid` tests, and feature negotiation tests.
vm/devices/virtio/virtio_net/Cargo.toml	Adds `tracelimit` dependency for rate-limited logging.
Cargo.lock	Records the new `tracelimit` dependency usage.

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Copilot

Pull request overview

Adds virtio-net TX/RX offload support (checksum + TSO/GSO) to improve networking performance while hardening malformed-guest handling.

Changes:

Advertise virtio-net offload features based on backend TxOffloadSupport and store negotiated features in the worker.
Parse virtio-net TX headers to populate TxMetadata offload flags/lengths; mark malformed TX packets as dropped (non-fatal) with ratelimited logging + new counter.
Set RX VIRTIO_NET_HDR_F_DATA_VALID based on backend checksum validation state; add extensive unit + integration-style tests.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
vm/devices/virtio/virtio_net/src/lib.rs	Feature advertisement, negotiated-feature plumbing, TX packet hardening + offload parsing, `tx_dropped` counter
vm/devices/virtio/virtio_net/src/buffers.rs	Sets RX `data_valid` header flag based on checksum validity
vm/devices/virtio/virtio_net/src/tests.rs	Adds offload parsing tests, RX `data_valid` tests, and feature negotiation tests; extends harness utilities
vm/devices/virtio/virtio_net/Cargo.toml	Adds `tracelimit` dependency for ratelimited TX-drop logging
Cargo.lock	Locks `tracelimit` dependency

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Copilot

Pull request overview

Adds virtio-net TX/RX offload support by negotiating and translating virtio-net header offload fields into backend TxMetadata, and by reflecting backend RX checksum validity back to the guest via VIRTIO_NET_HDR_F_DATA_VALID.

Changes:

Advertise checksum/TSO-related virtio-net features based on endpoint TxOffloadSupport, and gate TX offload parsing on negotiated features.
Harden TX/RX queue handling by dropping malformed/duplicate guest submissions non-fatally with rate-limited logging and new drop counters.
Extend unit and end-to-end tests to cover TX offload parsing, RX data_valid, duplicate descriptor handling, and feature negotiation.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
vm/devices/virtio/virtio_net/src/lib.rs	Feature advertisement, negotiated-feature plumbing into workers, TX offload parsing, and non-fatal drop handling with counters/logging.
vm/devices/virtio/virtio_net/src/buffers.rs	Map backend RX checksum states to virtio-net header `DATA_VALID`; handle duplicate RX descriptor submissions without panicking.
vm/devices/virtio/virtio_net/src/tests.rs	Add comprehensive TX offload parsing tests, RX `data_valid` tests, duplicate TX descriptor test, and feature negotiation tests.
vm/devices/virtio/virtio_net/Cargo.toml	Add `tracelimit` dependency for rate-limited guest-triggerable warnings.
Cargo.lock	Record the new `tracelimit` dependency in the lockfile.

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Copilot

Pull request overview

This PR adds virtio-net TX/RX offload support, translating virtio-net header offload fields into backend TxMetadata (checksum + TSO) and reporting RX checksum validation back to the guest via VIRTIO_NET_HDR_F_DATA_VALID.

Changes:

Advertise virtio-net offload feature bits based on backend TxOffloadSupport, and store negotiated features in each worker for per-packet gating.
Implement TX offload parsing (CSUM + TSO4/TSO6) with validation + non-fatal drop behavior and new dropped counters/logging.
Set RX data_valid based on backend checksum states; extend tests to cover offload parsing, end-to-end TX metadata, RX data_valid, and feature negotiation.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
vm/devices/virtio/virtio_net/src/lib.rs	Advertises offload features, stores negotiated features in workers, parses TX offloads, and hardens TX/RX queueing with drop handling + counters.
vm/devices/virtio/virtio_net/src/buffers.rs	Makes RX buffer queueing detect duplicates and sets RX `data_valid` header flag based on checksum validity.
vm/devices/virtio/virtio_net/src/tests.rs	Adds unit + integration tests covering TX offload parsing, end-to-end TX metadata, RX `data_valid`, and feature advertisement.
vm/devices/virtio/virtio_net/Cargo.toml	Adds `tracelimit` dependency for rate-limited drop logging.
Cargo.lock	Records the new `tracelimit` dependency usage.

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+                if l3_len > 0 && l4_len > 0 {
+                    flags.set_offload_tcp_segmentation(true);
+                    flags.set_offload_tcp_checksum(true);
+                    max_tcp_segment_size = header.gso_size;
+
+                    flags.set_is_ipv4(is_ipv4_from_gso);
+                    flags.set_is_ipv6(is_ipv6_from_gso);
+                    if is_ipv4_from_gso {


Copilot

Pull request overview

Adds virtio-net TX/RX offload support by translating virtio-net header offload fields into backend TxMetadata flags and by propagating backend RX checksum validity to the guest via VIRTIO_NET_HDR_F_DATA_VALID. The PR also hardens queue handling against malformed/duplicate guest submissions and expands test coverage around offload parsing and feature negotiation.

Changes:

Advertise virtio-net checksum/TSO features based on backend TxOffloadSupport, and gate offload parsing on negotiated features.
Implement TX offload parsing (CSUM + TSO4/TSO6) and RX DATA_VALID signaling; make malformed/duplicate packets non-fatal and counted as dropped.
Add unit + end-to-end tests for TX offloads, RX data_valid, duplicate descriptor handling, and feature negotiation.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
`vm/devices/virtio/virtio_net/src/lib.rs`	Advertises offload features, adds negotiated-feature gating, TX offload parsing, and drop-on-error hardening with new counters/logging.
`vm/devices/virtio/virtio_net/src/buffers.rs`	Handles duplicate RX descriptor submissions without panicking and sets `DATA_VALID` based on checksum validity.
`vm/devices/virtio/virtio_net/src/tests.rs`	Extends mock logging to capture TX metadata; adds extensive TX/RX offload and negotiation tests.
`vm/devices/virtio/virtio_net/Cargo.toml`	Adds `tracelimit` dependency for rate-limited drop logging.
`Cargo.lock`	Locks the added `tracelimit` dependency.

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github-actions · 2026-03-17T00:06:59Z

At least one Petri test failed.

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virtio-net all

9571fde

jstarks dismissed Brian-Perkins’s stale review via 9571fde March 16, 2026 21:57

jstarks force-pushed the virtio-net-offload branch from 366d80b to 9571fde Compare March 16, 2026 21:57

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jstarks enabled auto-merge (squash) March 16, 2026 23:01

jstarks disabled auto-merge March 16, 2026 23:52

jstarks merged commit 71e7202 into microsoft:main Mar 16, 2026
58 of 59 checks passed

jstarks deleted the virtio-net-offload branch March 16, 2026 23:53

damanm24 mentioned this pull request Mar 17, 2026

virtio_net: Add checksum offload support #2796

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jstarks commented Mar 13, 2026 •

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