bpf: Remove BPF_F_ARENA_SCALAR

Rust has no address spaces. Thus a prog in Rust accesses arena through
scalars. With BPF_F_ARENA_SCALAR the verifier accepts each load and each
store through a scalar as an access to arena.

The verifier rewrites one insn at a time. Thus it must add one insn to each
access. The compiler knows the source of each pointer. It can emit one
addr_space_cast for each pointer, as it does for the __arena pointers of C.

  flag: one move for each access     cast: one cast for each pointer

  ax = (u32)r1                       r1 = addr_space_cast(r1, 0, 1)
  r2 = *(u64 *)(ax + 0)              r2 = *(u64 *)(r1 + 0)
  ax = (u32)r1                       r3 = *(u64 *)(r1 + 8)
  r3 = *(u64 *)(ax + 8)

20 progs of rust-bpf, with the flag and with the casts that a prototype of
an LLVM pass emits:

                     flag    casts
  moves or casts     1568      571
  insns             14864    13938
  jited bytes       72351    69111

No released kernel has the flag. LLVM does not emit the casts for Rust at
this time. Until it does, Rust progs that access arena do not load.

scx_mavd and scx_nitosis set the flag too. They are experimental schedulers
in C in sched-ext/scx, and they use the flag to omit __arena. They must use
__arena again.

This reverts commit 4c651a91bdfc
("bpf: Treat load and store through a number as arena access"). One part
stays: BPF_REG_AX in reg2pt_regs[] of the x86 JIT. The fetching AND/OR/XOR
atomics in arena use it now.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261027190314.1440199-6-alexei.starovoitov@gmail.com

0 files changed, 0 insertions, 0 deletions