[PATCH bpf-next v2 0/2] Timed may_goto

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

This series replaces the current implementation of cond_break, which
uses the may_goto instruction, and counts 8 million iterations per stack
frame, with an implementation based on sampling time locally on the CPU.

This is done to permit a longer time for a given loop per-program
invocation. The accounting is still done per-stack frame, but the count
is used to instead amortize the cost of the logic to sample and check
the time spent since the start.

This is needed for expressing more complicated algorithms (spin locks,
waiting loops, etc.) in BPF programs without false positive expiration
of the loop. For instance, the plan is to make use of this for
implementing spin locks for BPF arena [0].

For the loop as follows:

for (int i = 0;; i++) {}

Testing on a bare-metal Sapphire Rapids Intel server yields the following
table (taking an average of 25 runs).

+-----------------------------+--------------+--------------+------------------+
| Loop type		      |	Iterations   |	Time (ms)   |	Time/iter (ns) |
+-----------------------------|--------------+--------------+------------------+
| may_goto		      |	8388608	     |	3	    |	0.36	       |
| timed_may_goto (count=65535)|	589674932    |	250	    |	0.42	       |
| bpf_for		      |	8388608	     |	10	    |	1.19	       |
+-----------------------------+--------------+--------------+------------------+

Here, count is used to amortize the time sampling and checking logic.

Obviously, this is the limit of an empty loop. Given the complexity of
the loop body, the time spent in the loop can be longer. Cancellations
will address the task of imposing an upper bound on program runtime.

For now, the implementation only supports x86.

  [0]: https://lore.kernel.org/bpf/20250118162238.2621311-1-memxor@gmail.com

Changelog:
----------
v1 -> v2
v1: https://lore.kernel.org/bpf/20250302201348.940234-1-memxor@gmail.com

 * Address comments from Alexei
   * Use kernel comment style for new code.
   * Remove p->count == 0 check in bpf_check_timed_may_goto.
   * Add comments on AX as argument/retval calling convention.
   * Add comments describing how the counting logic works.
   * Use BPF_EMIT_CALL instead of open-coding instruction encoding.
   * Change if ax != 1 goto pc+X condition to if ax != 0 goto pc+X.

Kumar Kartikeya Dwivedi (2):
  bpf: Add verifier support for timed may_goto
  bpf, x86: Add x86 JIT support for timed may_goto

 arch/x86/net/Makefile                         |  2 +-
 arch/x86/net/bpf_jit_comp.c                   |  5 ++
 arch/x86/net/bpf_timed_may_goto.S             | 52 ++++++++++++++
 include/linux/bpf.h                           |  1 +
 include/linux/filter.h                        |  8 +++
 kernel/bpf/core.c                             | 32 +++++++++
 kernel/bpf/verifier.c                         | 70 ++++++++++++++++---
 .../bpf/progs/verifier_bpf_fastcall.c         | 58 ++++++++++++---
 .../selftests/bpf/progs/verifier_may_goto_1.c | 34 ++++++++-
 9 files changed, 241 insertions(+), 21 deletions(-)
 create mode 100644 arch/x86/net/bpf_timed_may_goto.S


base-commit: 7586e2169c77a444d235a98ac858272d3dcec16e
-- 
2.43.5