Run Cloud Virtual Machines Securely and Efficiently

This release has been tracked in v51.0 group of our roadmap project.

Security Fixes #

This release fixes a security vulnerability in disk image handling. Details can be found in GHSA-jmr4-g2hv-mjj6.

• A new backing_files=on|off option has been added to --disk to explicitly control whether QCOW2 backing files are permitted. This defaults to off to prevent the loading of backing files entirely. (#7685)
• Explicit image type specification via the user interface, removing reliance on format autodetection (#7728).
• Prevent sector-zero writes for autodetected raw images (#7728).

Significant QCOW2 v3 Improvements #

A large number of QCOW2 v3 specification features have been implemented:

• RAW backing file support for QCOW2 overlays (#7570)
• Zero bit in L2 entries (#7627)
• Incompatible feature bit validation (#7612)
• Dirty bit support (#7636)
• Variable refcount widths (1 to 64-bit) (#7633)
• Corrupt bit detection and marking (#7639)
• Autoclear feature bits handling (#7648)
• Thread safety fix for multiple virtio queues (num_queues > 1) (#7661)
• Correct zero-fill for reads beyond backing file size (#7678)
• Live disk resize support (#7687)

ACPI Generic Initiator Support #

ACPI Generic Initiator Affinity (SRAT Type 5) support has been added to associate VFIO-PCI devices with dedicated memory/CPU-less NUMA nodes. This enables the guest OS to make NUMA-aware memory allocation decisions for device workloads. A new device_id parameter has been added to --numa for specifying VFIO devices. (#7626)

Block Device DISCARD and WRITE_ZEROES Support #

The virtio-blk device now supports DISCARD and WRITE_ZEROES operations for QCOW2 and RAW image formats. This enables thin provisioning and efficient space reclamation when guests trim filesystems. A new sparse=on|off option has been added to --disk to control disk space management: sparse=on (default) enables thin provisioning with space reclamation, while sparse=off provides thick provisioning with consistent I/O latency. (#7666)

Notable Performance Improvements #

• Transparent Huge Pages (THP) support has been extended to cover anonymous shared memory (shared=on) via madvise. Previously, THP was only used for non-shared memory. (#7646)
• The vhost-user-net device now uses the default set of vhost-user virtio features, including VIRTIO_F_RING_INDIRECT_DESC, which provides a performance improvement. (#7653)

MSHV Support Improvements #

• Optimize CPU state update after emulation by only updating special registers when changed (#7603)
• Enable SMT for guests with threads_per_core > 1 (#7668)
• Stub save_data_tables() to unblock VM pause/resume (#7692)
• Handle GHCB_INFO_SPECIAL_DBGPRINT VMG exit in SEV-SNP guest exit handler (#7703)
• Fix CVM boot failure on MSHV (#7548)
• Fix CPU topology detection for multithreaded configurations (#7576)

Notable Bug Fixes #

• Fix VFIO device hot-remove leaving group and container file descriptors open, preventing re-add (#7676)
• Fix snapshot restore when backing file is on read-only storage with shared=false (#7674)
• Enforce VIRTIO_BLK_F_RO even if guest does not negotiate it (#7705)
• Fix read-only block device FLUSH requests from OVMF preventing VMs from booting (#7706)
• Fix vhost-user device not properly dropping unowned file descriptors (#7679)
• Fix vhost-user-block get_config interoperability (#7617)
• Fix vsock TOCTOU race condition by copying packet header from guest memory before processing (#7530)
• Fix vsock handling of large TX packets spanning multiple data descriptors (#7680)
• Add gettid() to all seccomp filters (#7596)
• Fix MAC address parsing that wrongly allowed + instead of hex characters (#7579)
• Improve UUID parse error message and --net fd help text (#7702)
• Fix various inconsistencies in our OpenAPI specification file (#7716, #7726)
• Various documentation fixes (#7602, #7606)

Contributors #

Many thanks to everyone who has contributed to our release:

• Aastha Rawat aastharawat@microsoft.com
• Alyssa Ross hi@alyssa.is
• Anatol Belski anbelski@linux.microsoft.com
• Anirudh Rayabharam anrayabh@microsoft.com
• Bo Chen bchen@crusoe.ai
• Champ-Goblem cameron@northflank.com
• Changyuan Lyu changyuanl@google.com
• Damian Barabonkov dbctl@pm.me
• Demi Marie Obenour demiobenour@gmail.com
• Leander Kohler leander.kohler@cyberus-technology.de
• Muminul Islam muislam@microsoft.com
• Philipp Schuster philipp.schuster@cyberus-technology.de
• Rob Bradford rbradford@meta.com
• Rowen-Ye rowenye1@gmail.com
• Saravanan D saravanand@crusoe.ai
• Stanislav Kinsburskii skinsburskii@linux.microsoft.com
• Thomas Leroy thomas.leroy.mp@gmail.com
• Wei Liu liuwe@microsoft.com
• Yi Wang foxywang@tencent.com
• Zhibin Li banlu.lzb@antgroup.com
• stevenhorsman steven@uk.ibm.com

Download #

See the GitHub Release for the release assets.