5.3 Merge window, part 2

Some of the more significant changes merged in the second half of the merge window are:

Architecture-specific

• User-mode Linux now has a "time-travel mode" that can speed up or slow down time to help with debugging of timing-related code.
• The RISC-V architecture now has huge-page support.

Core kernel

• The io_uring mechanism has gained support for asynchronous sendmsg() and recvmsg() operations.
• Kernel probes ("kprobes") have gained the ability to dereference pointers into user space. This can be used, for example, to look inside structures passed into system calls. Some documentation can be found in this commit. It is also now possible to set kprobes at boot time.
• There is a new configuration option (PREEMPT_RT) to select realtime preemption. The code to implement that selection is not yet upstream, but this is a signal that said code is coming soon.

Filesystems and block layer

• The ubifs filesystem has gained support for zstd compression and verification of offline-signed filesystem images.
• The Ceph filesystem now supports security labels on files and, thus, now works with SELinux.
• There is a new mount option (nconnect=) for NFSv4 filesystems that allows the administrator to request that multiple TCP connections be established to the server. Traffic is then load-balanced across those connections.

Hardware support

• Graphics: Samsung S6E63M0 RGB/SPI panels, OSD OSD101T2587-53TS DSI 1920x1200 video mode panels, ST-Ericsson multichannel display engines, and AMD NAVI GPUs.
• Miscellaneous: NXP IMX8MN pin controllers, Qualcomm SM8150 pin controllers, Cypress HyperBus-connected devices, ROHM BD70528 power-management ICs, Rockchip RK809 and RK817 power-management ICs Cirrus Logic CS47L15, CS42L92, CS47L92 and CS47L93 codecs, MediaTek MT7621/MT7628 I2C controllers, SiLabs 5341 and 5340 clock generators, and Apple SPI keyboards and trackpads.

Security-related

• There are two new command-line options, init_on_alloc= and init_on_free=, that will cause memory allocations to be initialized to zero at allocation and free time, respectively. They improve security at some runtime cost, and are expected to be enabled on Android and ChromeOS systems. See this commit for details.
• There is a new event filtering mechanism for perf events under KVM on the x86 architecture; it allows an administrator to specify which types of events can (or cannot) be monitored by guests. Some details can be found in this commit.

Virtualization and containers

• The new virtio-iommu driver emulates an I/O memory-management unit on top of the virtio mechanism.
• There is a new virtio-pmem driver that emulates persistent memory devices in the guest.

Internal kernel changes

• Kernel makefiles can now contain a header-test-y declaration specifying header files that should be able to be compiled on their own; this assertion will be tested at build time if the HEADER_TEST configuration variable is set. See this commit for details. The plan is to test all kernel headers this way, but many have to be excluded in current kernels.
• The conversion of kernel documentation to the ReStructured Text format has jumped forward with this massive patch set taking care of most of the remaining plain-text files. Now that the formatting issues are mostly taken care of, all that is left is the trivial task of integrating the documentation and updating it to match the current state of the kernel.
• The venerable floppy driver has been marked as orphaned, since the last remaining maintainer no longer has a working drive.

Now comes the task of stabilizing all of that work. If the usual schedule holds — and it almost always does these days — the final 5.3 release can be expected on September 8 or 15.