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.
  (vmm: Add option to control backing files #7685)
• Explicit image type specification via the user interface, removing
  reliance on format autodetection (Add more resiliency to disk image format handling #7728).
• Prevent sector-zero writes for autodetected raw images (Add more resiliency to disk image format handling #7728).

Significant QCOW2 v3 Improvements

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

• RAW backing file support for QCOW2 overlays (block: qcow: Add RAW backing file support for QCOW2 images #7570)
• Zero bit in L2 entries (block: qcow: Add read support for zero bit in L2 entries #7627)
• Incompatible feature bit validation (block: qcow: Validate QCOW2 v3 incompatible feature bits #7612)
• Dirty bit support (block: qcow: Implement QCOW2 v3 dirty bit support #7636)
• Variable refcount widths (1 to 64-bit) (block: qcow: Support variable refcount widths #7633)
• Corrupt bit detection and marking (block: qcow: Implement QCOW2 v3 corrupt bit support #7639)
• Autoclear feature bits handling (block: qcow: Clear QCOW2 v3 autoclear feature bits on writable open #7648)
• Thread safety fix for multiple virtio queues (num_queues > 1)
  (block: qcow: Fix QCOW2 thread safety for multiple virtio queues #7661)
• Correct zero-fill for reads beyond backing file size (block: qcow: Return zeros for reads beyond backing file size #7678)
• Live disk resize support (block: qcow: Add 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. (vmm: enable THP for anon shmem #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. (virtio-devices: vhost-user: net: Use default vhost-user virtio features #7653)

MSHV Support Improvements

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

Notable Bug Fixes

• Fix VFIO device hot-remove leaving group and container file
  descriptors open, preventing re-add (fix(vfio): fix VFIO device hot-remove fd leak preventing re-add #7676)
• Fix snapshot restore when backing file is on read-only storage with
  shared=false (vmm: open backing file read-only when not shared #7674)
• Enforce VIRTIO_BLK_F_RO even if guest does not negotiate it
  (block: do not allow guest to not negotiate VIRTIO_BLK_F_RO #7705)
• Fix read-only block device FLUSH requests from OVMF preventing VMs
  from booting (block: Allow (and ignore) FLUSH requests on read-only devices #7706)
• Fix vhost-user device not properly dropping unowned file descriptors
  (virtio-devices: Do not close an unowned FD #7679)
• Fix vhost-user-block get_config interoperability (vhost-user-block: Fix get_config interopability #7617)
• Fix vsock TOCTOU race condition by copying packet header from guest
  memory before processing (virtio-devices: copy VSock header from guest #7530)
• Fix vsock handling of large TX packets spanning multiple data
  descriptors (Accept multi-descriptor TX packets in vsock #7680)
• Add gettid() to all seccomp filters (Add gettid() to all seccomp filters #7596)
• Fix MAC address parsing that wrongly allowed + instead of hex
  characters (net_util: Fix MAC address parsing #7579)
• Improve UUID parse error message and --net fd help text (vmm: improve UUID parse err and --net fd help #7702)
• Fix various inconsistencies in our OpenAPI specification file
  (vmm: api: Expose the nested option in API description #7716, vmm: openapi: Sync DiskConfig OpenAPI spec #7726)
• Various documentation fixes (Documentation fixes #7602, docs: fix gdb path option name #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