Merge tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-nextgrafted

Pull networking updates from Jakub Kicinski:
 "Core:

   - Add dedicated kmem_cache for typical/small skb->head, avoid having
     to access struct page at kfree time, and improve memory use.

   - Introduce sysctl to set default RPS configuration for new netdevs.

   - Define Netlink protocol specification format which can be used to
     describe messages used by each family and auto-generate parsers.
     Add tools for generating kernel data structures and uAPI headers.

   - Expose all net/core sysctls inside netns.

   - Remove 4s sleep in netpoll if carrier is instantly detected on
     boot.

   - Add configurable limit of MDB entries per port, and port-vlan.

   - Continue populating drop reasons throughout the stack.

   - Retire a handful of legacy Qdiscs and classifiers.

  Protocols:

   - Support IPv4 big TCP (TSO frames larger than 64kB).

   - Add IP_LOCAL_PORT_RANGE socket option, to control local port range
     on socket by socket basis.

   - Track and report in procfs number of MPTCP sockets used.

   - Support mixing IPv4 and IPv6 flows in the in-kernel MPTCP path
     manager.

   - IPv6: don't check net.ipv6.route.max_size and rely on garbage
     collection to free memory (similarly to IPv4).

   - Support Penultimate Segment Pop (PSP) flavor in SRv6 (RFC8986).

   - ICMP: add per-rate limit counters.

   - Add support for user scanning requests in ieee802154.

   - Remove static WEP support.

   - Support minimal Wi-Fi 7 Extremely High Throughput (EHT) rate
     reporting.

   - WiFi 7 EHT channel puncturing support (client & AP).

  BPF:

   - Add a rbtree data structure following the "next-gen data structure"
     precedent set by recently added linked list, that is, by using
     kfunc + kptr instead of adding a new BPF map type.

   - Expose XDP hints via kfuncs with initial support for RX hash and
     timestamp metadata.

   - Add BPF_F_NO_TUNNEL_KEY extension to bpf_skb_set_tunnel_key to
     better support decap on GRE tunnel devices not operating in collect
     metadata.

   - Improve x86 JIT's codegen for PROBE_MEM runtime error checks.

   - Remove the need for trace_printk_lock for bpf_trace_printk and
     bpf_trace_vprintk helpers.

   - Extend libbpf's bpf_tracing.h support for tracing arguments of
     kprobes/uprobes and syscall as a special case.

   - Significantly reduce the search time for module symbols by
     livepatch and BPF.

   - Enable cpumasks to be used as kptrs, which is useful for tracing
     programs tracking which tasks end up running on which CPUs in
     different time intervals.

   - Add support for BPF trampoline on s390x and riscv64.

   - Add capability to export the XDP features supported by the NIC.

   - Add __bpf_kfunc tag for marking kernel functions as kfuncs.

   - Add cgroup.memory=nobpf kernel parameter option to disable BPF
     memory accounting for container environments.

  Netfilter:

   - Remove the CLUSTERIP target. It has been marked as obsolete for
     years, and we still have WARN splats wrt races of the out-of-band
     /proc interface installed by this target.

   - Add 'destroy' commands to nf_tables. They are identical to the
     existing 'delete' commands, but do not return an error if the
     referenced object (set, chain, rule...) did not exist.

  Driver API:

   - Improve cpumask_local_spread() locality to help NICs set the right
     IRQ affinity on AMD platforms.

   - Separate C22 and C45 MDIO bus transactions more clearly.

   - Introduce new DCB table to control DSCP rewrite on egress.

   - Support configuration of Physical Layer Collision Avoidance (PLCA)
     Reconciliation Sublayer (RS) (802.3cg-2019). Modern version of
     shared medium Ethernet.

   - Support for MAC Merge layer (IEEE 802.3-2018 clause 99). Allowing
     preemption of low priority frames by high priority frames.

   - Add support for controlling MACSec offload using netlink SET.

   - Rework devlink instance refcounts to allow registration and
     de-registration under the instance lock. Split the code into
     multiple files, drop some of the unnecessarily granular locks and
     factor out common parts of netlink operation handling.

   - Add TX frame aggregation parameters (for USB drivers).

   - Add a new attr TCA_EXT_WARN_MSG to report TC (offload) warning
     messages with notifications for debug.

   - Allow offloading of UDP NEW connections via act_ct.

   - Add support for per action HW stats in TC.

   - Support hardware miss to TC action (continue processing in SW from
     a specific point in the action chain).

   - Warn if old Wireless Extension user space interface is used with
     modern cfg80211/mac80211 drivers. Do not support Wireless
     Extensions for Wi-Fi 7 devices at all. Everyone should switch to
     using nl80211 interface instead.

   - Improve the CAN bit timing configuration. Use extack to return
     error messages directly to user space, update the SJW handling,
     including the definition of a new default value that will benefit
     CAN-FD controllers, by increasing their oscillator tolerance.

  New hardware / drivers:

   - Ethernet:
      - nVidia BlueField-3 support (control traffic driver)
      - Ethernet support for imx93 SoCs
      - Motorcomm yt8531 gigabit Ethernet PHY
      - onsemi NCN26000 10BASE-T1S PHY (with support for PLCA)
      - Microchip LAN8841 PHY (incl. cable diagnostics and PTP)
      - Amlogic gxl MDIO mux

   - WiFi:
      - RealTek RTL8188EU (rtl8xxxu)
      - Qualcomm Wi-Fi 7 devices (ath12k)

   - CAN:
      - Renesas R-Car V4H

  Drivers:

   - Bluetooth:
      - Set Per Platform Antenna Gain (PPAG) for Intel controllers.

   - Ethernet NICs:
      - Intel (1G, igc):
         - support TSN / Qbv / packet scheduling features of i226 model
      - Intel (100G, ice):
         - use GNSS subsystem instead of TTY
         - multi-buffer XDP support
         - extend support for GPIO pins to E823 devices
      - nVidia/Mellanox:
         - update the shared buffer configuration on PFC commands
         - implement PTP adjphase function for HW offset control
         - TC support for Geneve and GRE with VF tunnel offload
         - more efficient crypto key management method
         - multi-port eswitch support
      - Netronome/Corigine:
         - add DCB IEEE support
         - support IPsec offloading for NFP3800
      - Freescale/NXP (enetc):
         - support XDP_REDIRECT for XDP non-linear buffers
         - improve reconfig, avoid link flap and waiting for idle
         - support MAC Merge layer
      - Other NICs:
         - sfc/ef100: add basic devlink support for ef100
         - ionic: rx_push mode operation (writing descriptors via MMIO)
         - bnxt: use the auxiliary bus abstraction for RDMA
         - r8169: disable ASPM and reset bus in case of tx timeout
         - cpsw: support QSGMII mode for J721e CPSW9G
         - cpts: support pulse-per-second output
         - ngbe: add an mdio bus driver
         - usbnet: optimize usbnet_bh() by avoiding unnecessary queuing
         - r8152: handle devices with FW with NCM support
         - amd-xgbe: support 10Mbps, 2.5GbE speeds and rx-adaptation
         - virtio-net: support multi buffer XDP
         - virtio/vsock: replace virtio_vsock_pkt with sk_buff
         - tsnep: XDP support

   - Ethernet high-speed switches:
      - nVidia/Mellanox (mlxsw):
         - add support for latency TLV (in FW control messages)
      - Microchip (sparx5):
         - separate explicit and implicit traffic forwarding rules, make
           the implicit rules always active
         - add support for egress DSCP rewrite
         - IS0 VCAP support (Ingress Classification)
         - IS2 VCAP filters (protos, L3 addrs, L4 ports, flags, ToS
           etc.)
         - ES2 VCAP support (Egress Access Control)
         - support for Per-Stream Filtering and Policing (802.1Q,
           8.6.5.1)

   - Ethernet embedded switches:
      - Marvell (mv88e6xxx):
         - add MAB (port auth) offload support
         - enable PTP receive for mv88e6390
      - NXP (ocelot):
         - support MAC Merge layer
         - support for the the vsc7512 internal copper phys
      - Microchip:
         - lan9303: convert to PHYLINK
         - lan966x: support TC flower filter statistics
         - lan937x: PTP support for KSZ9563/KSZ8563 and LAN937x
         - lan937x: support Credit Based Shaper configuration
         - ksz9477: support Energy Efficient Ethernet
      - other:
         - qca8k: convert to regmap read/write API, use bulk operations
         - rswitch: Improve TX timestamp accuracy

   - Intel WiFi (iwlwifi):
      - EHT (Wi-Fi 7) rate reporting
      - STEP equalizer support: transfer some STEP (connection to radio
        on platforms with integrated wifi) related parameters from the
        BIOS to the firmware.

   - Qualcomm 802.11ax WiFi (ath11k):
      - IPQ5018 support
      - Fine Timing Measurement (FTM) responder role support
      - channel 177 support

   - MediaTek WiFi (mt76):
      - per-PHY LED support
      - mt7996: EHT (Wi-Fi 7) support
      - Wireless Ethernet Dispatch (WED) reset support
      - switch to using page pool allocator

   - RealTek WiFi (rtw89):
      - support new version of Bluetooth co-existance

   - Mobile:
      - rmnet: support TX aggregation"

* tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next: (1872 commits)
  page_pool: add a comment explaining the fragment counter usage
  net: ethtool: fix __ethtool_dev_mm_supported() implementation
  ethtool: pse-pd: Fix double word in comments
  xsk: add linux/vmalloc.h to xsk.c
  sefltests: netdevsim: wait for devlink instance after netns removal
  selftest: fib_tests: Always cleanup before exit
  net/mlx5e: Align IPsec ASO result memory to be as required by hardware
  net/mlx5e: TC, Set CT miss to the specific ct action instance
  net/mlx5e: Rename CHAIN_TO_REG to MAPPED_OBJ_TO_REG
  net/mlx5: Refactor tc miss handling to a single function
  net/mlx5: Kconfig: Make tc offload depend on tc skb extension
  net/sched: flower: Support hardware miss to tc action
  net/sched: flower: Move filter handle initialization earlier
  net/sched: cls_api: Support hardware miss to tc action
  net/sched: Rename user cookie and act cookie
  sfc: fix builds without CONFIG_RTC_LIB
  sfc: clean up some inconsistent indentings
  net/mlx4_en: Introduce flexible array to silence overflow warning
  net: lan966x: Fix possible deadlock inside PTP
  net/ulp: Remove redundant ->clone() test in inet_clone_ulp().
  ...

1 files changed, 356 insertions, 0 deletions

diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst
new file mode 100644
index 000000000..bfff0e7e3
--- /dev/null
+++ b/Documentation/bpf/bpf_design_QA.rst
@@ -0,0 +1,356 @@
+==============
+BPF Design Q&A
+==============
+
+BPF extensibility and applicability to networking, tracing, security
+in the linux kernel and several user space implementations of BPF
+virtual machine led to a number of misunderstanding on what BPF actually is.
+This short QA is an attempt to address that and outline a direction
+of where BPF is heading long term.
+
+.. contents::
+    :local:
+    :depth: 3
+
+Questions and Answers
+=====================
+
+Q: Is BPF a generic instruction set similar to x64 and arm64?
+-------------------------------------------------------------
+A: NO.
+
+Q: Is BPF a generic virtual machine ?
+-------------------------------------
+A: NO.
+
+BPF is generic instruction set *with* C calling convention.
+-----------------------------------------------------------
+
+Q: Why C calling convention was chosen?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A: Because BPF programs are designed to run in the linux kernel
+which is written in C, hence BPF defines instruction set compatible
+with two most used architectures x64 and arm64 (and takes into
+consideration important quirks of other architectures) and
+defines calling convention that is compatible with C calling
+convention of the linux kernel on those architectures.
+
+Q: Can multiple return values be supported in the future?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: NO. BPF allows only register R0 to be used as return value.
+
+Q: Can more than 5 function arguments be supported in the future?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: NO. BPF calling convention only allows registers R1-R5 to be used
+as arguments. BPF is not a standalone instruction set.
+(unlike x64 ISA that allows msft, cdecl and other conventions)
+
+Q: Can BPF programs access instruction pointer or return address?
+-----------------------------------------------------------------
+A: NO.
+
+Q: Can BPF programs access stack pointer ?
+------------------------------------------
+A: NO.
+
+Only frame pointer (register R10) is accessible.
+From compiler point of view it's necessary to have stack pointer.
+For example, LLVM defines register R11 as stack pointer in its
+BPF backend, but it makes sure that generated code never uses it.
+
+Q: Does C-calling convention diminishes possible use cases?
+-----------------------------------------------------------
+A: YES.
+
+BPF design forces addition of major functionality in the form
+of kernel helper functions and kernel objects like BPF maps with
+seamless interoperability between them. It lets kernel call into
+BPF programs and programs call kernel helpers with zero overhead,
+as all of them were native C code. That is particularly the case
+for JITed BPF programs that are indistinguishable from
+native kernel C code.
+
+Q: Does it mean that 'innovative' extensions to BPF code are disallowed?
+------------------------------------------------------------------------
+A: Soft yes.
+
+At least for now, until BPF core has support for
+bpf-to-bpf calls, indirect calls, loops, global variables,
+jump tables, read-only sections, and all other normal constructs
+that C code can produce.
+
+Q: Can loops be supported in a safe way?
+----------------------------------------
+A: It's not clear yet.
+
+BPF developers are trying to find a way to
+support bounded loops.
+
+Q: What are the verifier limits?
+--------------------------------
+A: The only limit known to the user space is BPF_MAXINSNS (4096).
+It's the maximum number of instructions that the unprivileged bpf
+program can have. The verifier has various internal limits.
+Like the maximum number of instructions that can be explored during
+program analysis. Currently, that limit is set to 1 million.
+Which essentially means that the largest program can consist
+of 1 million NOP instructions. There is a limit to the maximum number
+of subsequent branches, a limit to the number of nested bpf-to-bpf
+calls, a limit to the number of the verifier states per instruction,
+a limit to the number of maps used by the program.
+All these limits can be hit with a sufficiently complex program.
+There are also non-numerical limits that can cause the program
+to be rejected. The verifier used to recognize only pointer + constant
+expressions. Now it can recognize pointer + bounded_register.
+bpf_lookup_map_elem(key) had a requirement that 'key' must be
+a pointer to the stack. Now, 'key' can be a pointer to map value.
+The verifier is steadily getting 'smarter'. The limits are
+being removed. The only way to know that the program is going to
+be accepted by the verifier is to try to load it.
+The bpf development process guarantees that the future kernel
+versions will accept all bpf programs that were accepted by
+the earlier versions.
+
+
+Instruction level questions
+---------------------------
+
+Q: LD_ABS and LD_IND instructions vs C code
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Q: How come LD_ABS and LD_IND instruction are present in BPF whereas
+C code cannot express them and has to use builtin intrinsics?
+
+A: This is artifact of compatibility with classic BPF. Modern
+networking code in BPF performs better without them.
+See 'direct packet access'.
+
+Q: BPF instructions mapping not one-to-one to native CPU
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Q: It seems not all BPF instructions are one-to-one to native CPU.
+For example why BPF_JNE and other compare and jumps are not cpu-like?
+
+A: This was necessary to avoid introducing flags into ISA which are
+impossible to make generic and efficient across CPU architectures.
+
+Q: Why BPF_DIV instruction doesn't map to x64 div?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: Because if we picked one-to-one relationship to x64 it would have made
+it more complicated to support on arm64 and other archs. Also it
+needs div-by-zero runtime check.
+
+Q: Why there is no BPF_SDIV for signed divide operation?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: Because it would be rarely used. llvm errors in such case and
+prints a suggestion to use unsigned divide instead.
+
+Q: Why BPF has implicit prologue and epilogue?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: Because architectures like sparc have register windows and in general
+there are enough subtle differences between architectures, so naive
+store return address into stack won't work. Another reason is BPF has
+to be safe from division by zero (and legacy exception path
+of LD_ABS insn). Those instructions need to invoke epilogue and
+return implicitly.
+
+Q: Why BPF_JLT and BPF_JLE instructions were not introduced in the beginning?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+A: Because classic BPF didn't have them and BPF authors felt that compiler
+workaround would be acceptable. Turned out that programs lose performance
+due to lack of these compare instructions and they were added.
+These two instructions is a perfect example what kind of new BPF
+instructions are acceptable and can be added in the future.
+These two already had equivalent instructions in native CPUs.
+New instructions that don't have one-to-one mapping to HW instructions
+will not be accepted.
+
+Q: BPF 32-bit subregister requirements
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Q: BPF 32-bit subregisters have a requirement to zero upper 32-bits of BPF
+registers which makes BPF inefficient virtual machine for 32-bit
+CPU architectures and 32-bit HW accelerators. Can true 32-bit registers
+be added to BPF in the future?
+
+A: NO.
+
+But some optimizations on zero-ing the upper 32 bits for BPF registers are
+available, and can be leveraged to improve the performance of JITed BPF
+programs for 32-bit architectures.
+
+Starting with version 7, LLVM is able to generate instructions that operate
+on 32-bit subregisters, provided the option -mattr=+alu32 is passed for
+compiling a program. Furthermore, the verifier can now mark the
+instructions for which zero-ing the upper bits of the destination register
+is required, and insert an explicit zero-extension (zext) instruction
+(a mov32 variant). This means that for architectures without zext hardware
+support, the JIT back-ends do not need to clear the upper bits for
+subregisters written by alu32 instructions or narrow loads. Instead, the
+back-ends simply need to support code generation for that mov32 variant,
+and to overwrite bpf_jit_needs_zext() to make it return "true" (in order to
+enable zext insertion in the verifier).
+
+Note that it is possible for a JIT back-end to have partial hardware
+support for zext. In that case, if verifier zext insertion is enabled,
+it could lead to the insertion of unnecessary zext instructions. Such
+instructions could be removed by creating a simple peephole inside the JIT
+back-end: if one instruction has hardware support for zext and if the next
+instruction is an explicit zext, then the latter can be skipped when doing
+the code generation.
+
+Q: Does BPF have a stable ABI?
+------------------------------
+A: YES. BPF instructions, arguments to BPF programs, set of helper
+functions and their arguments, recognized return codes are all part
+of ABI. However there is one specific exception to tracing programs
+which are using helpers like bpf_probe_read() to walk kernel internal
+data structures and compile with kernel internal headers. Both of these
+kernel internals are subject to change and can break with newer kernels
+such that the program needs to be adapted accordingly.
+
+New BPF functionality is generally added through the use of kfuncs instead of
+new helpers. Kfuncs are not considered part of the stable API, and have their own
+lifecycle expectations as described in :ref:`BPF_kfunc_lifecycle_expectations`.
+
+Q: Are tracepoints part of the stable ABI?
+------------------------------------------
+A: NO. Tracepoints are tied to internal implementation details hence they are
+subject to change and can break with newer kernels. BPF programs need to change
+accordingly when this happens.
+
+Q: Are places where kprobes can attach part of the stable ABI?
+--------------------------------------------------------------
+A: NO. The places to which kprobes can attach are internal implementation
+details, which means that they are subject to change and can break with
+newer kernels. BPF programs need to change accordingly when this happens.
+
+Q: How much stack space a BPF program uses?
+-------------------------------------------
+A: Currently all program types are limited to 512 bytes of stack
+space, but the verifier computes the actual amount of stack used
+and both interpreter and most JITed code consume necessary amount.
+
+Q: Can BPF be offloaded to HW?
+------------------------------
+A: YES. BPF HW offload is supported by NFP driver.
+
+Q: Does classic BPF interpreter still exist?
+--------------------------------------------
+A: NO. Classic BPF programs are converted into extend BPF instructions.
+
+Q: Can BPF call arbitrary kernel functions?
+-------------------------------------------
+A: NO. BPF programs can only call specific functions exposed as BPF helpers or
+kfuncs. The set of available functions is defined for every program type.
+
+Q: Can BPF overwrite arbitrary kernel memory?
+---------------------------------------------
+A: NO.
+
+Tracing bpf programs can *read* arbitrary memory with bpf_probe_read()
+and bpf_probe_read_str() helpers. Networking programs cannot read
+arbitrary memory, since they don't have access to these helpers.
+Programs can never read or write arbitrary memory directly.
+
+Q: Can BPF overwrite arbitrary user memory?
+-------------------------------------------
+A: Sort-of.
+
+Tracing BPF programs can overwrite the user memory
+of the current task with bpf_probe_write_user(). Every time such
+program is loaded the kernel will print warning message, so
+this helper is only useful for experiments and prototypes.
+Tracing BPF programs are root only.
+
+Q: New functionality via kernel modules?
+----------------------------------------
+Q: Can BPF functionality such as new program or map types, new
+helpers, etc be added out of kernel module code?
+
+A: Yes, through kfuncs and kptrs
+
+The core BPF functionality such as program types, maps and helpers cannot be
+added to by modules. However, modules can expose functionality to BPF programs
+by exporting kfuncs (which may return pointers to module-internal data
+structures as kptrs).
+
+Q: Directly calling kernel function is an ABI?
+----------------------------------------------
+Q: Some kernel functions (e.g. tcp_slow_start) can be called
+by BPF programs.  Do these kernel functions become an ABI?
+
+A: NO.
+
+The kernel function protos will change and the bpf programs will be
+rejected by the verifier.  Also, for example, some of the bpf-callable
+kernel functions have already been used by other kernel tcp
+cc (congestion-control) implementations.  If any of these kernel
+functions has changed, both the in-tree and out-of-tree kernel tcp cc
+implementations have to be changed.  The same goes for the bpf
+programs and they have to be adjusted accordingly. See
+:ref:`BPF_kfunc_lifecycle_expectations` for details.
+
+Q: Attaching to arbitrary kernel functions is an ABI?
+-----------------------------------------------------
+Q: BPF programs can be attached to many kernel functions.  Do these
+kernel functions become part of the ABI?
+
+A: NO.
+
+The kernel function prototypes will change, and BPF programs attaching to
+them will need to change.  The BPF compile-once-run-everywhere (CO-RE)
+should be used in order to make it easier to adapt your BPF programs to
+different versions of the kernel.
+
+Q: Marking a function with BTF_ID makes that function an ABI?
+-------------------------------------------------------------
+A: NO.
+
+The BTF_ID macro does not cause a function to become part of the ABI
+any more than does the EXPORT_SYMBOL_GPL macro.
+
+Q: What is the compatibility story for special BPF types in map values?
+-----------------------------------------------------------------------
+Q: Users are allowed to embed bpf_spin_lock, bpf_timer fields in their BPF map
+values (when using BTF support for BPF maps). This allows to use helpers for
+such objects on these fields inside map values. Users are also allowed to embed
+pointers to some kernel types (with __kptr and __kptr_ref BTF tags). Will the
+kernel preserve backwards compatibility for these features?
+
+A: It depends. For bpf_spin_lock, bpf_timer: YES, for kptr and everything else:
+NO, but see below.
+
+For struct types that have been added already, like bpf_spin_lock and bpf_timer,
+the kernel will preserve backwards compatibility, as they are part of UAPI.
+
+For kptrs, they are also part of UAPI, but only with respect to the kptr
+mechanism. The types that you can use with a __kptr and __kptr_ref tagged
+pointer in your struct are NOT part of the UAPI contract. The supported types can
+and will change across kernel releases. However, operations like accessing kptr
+fields and bpf_kptr_xchg() helper will continue to be supported across kernel
+releases for the supported types.
+
+For any other supported struct type, unless explicitly stated in this document
+and added to bpf.h UAPI header, such types can and will arbitrarily change their
+size, type, and alignment, or any other user visible API or ABI detail across
+kernel releases. The users must adapt their BPF programs to the new changes and
+update them to make sure their programs continue to work correctly.
+
+NOTE: BPF subsystem specially reserves the 'bpf\_' prefix for type names, in
+order to introduce more special fields in the future. Hence, user programs must
+avoid defining types with 'bpf\_' prefix to not be broken in future releases.
+In other words, no backwards compatibility is guaranteed if one using a type
+in BTF with 'bpf\_' prefix.
+
+Q: What is the compatibility story for special BPF types in allocated objects?
+------------------------------------------------------------------------------
+Q: Same as above, but for allocated objects (i.e. objects allocated using
+bpf_obj_new for user defined types). Will the kernel preserve backwards
+compatibility for these features?
+
+A: NO.
+
+Unlike map value types, the API to work with allocated objects and any support
+for special fields inside them is exposed through kfuncs, and thus has the same
+lifecycle expectations as the kfuncs themselves. See
+:ref:`BPF_kfunc_lifecycle_expectations` for details.