Cloud Hypervisor v51.0 Released!

Posted February 20, 2026 by Cloud Hypervisor Team ‐ 3 min read

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:

Download

See the GitHub Release for the release assets.