Re: [PATCH bpf-next v3 2/4] arm64: insn: add encoders for atomic operations

Hi,

On 2/17/2022 1:16 AM, Will Deacon wrote:

On Fri, Feb 11, 2022 at 03:39:48PM +0100, Daniel Borkmann wrote:

quoted

On 1/29/22 11:04 PM, Hou Tao wrote:

quoted

It is a preparation patch for eBPF atomic supports under arm64. eBPF
needs support atomic[64]_fetch_add, atomic[64]_[fetch_]{and,or,xor} and
atomic[64]_{xchg|cmpxchg}. The ordering semantics of eBPF atomics are
the same with the implementations in linux kernel.

Add three helpers to support LDCLR/LDEOR/LDSET/SWP, CAS and DMB
instructions. STADD/STCLR/STEOR/STSET are simply encoded as aliases for
LDADD/LDCLR/LDEOR/LDSET with XZR as the destination register, so no extra
helper is added. atomic_fetch_add() and other atomic ops needs support for
STLXR instruction, so extend enum aarch64_insn_ldst_type to do that.

LDADD/LDEOR/LDSET/SWP and CAS instructions are only available when LSE
atomics is enabled, so just return AARCH64_BREAK_FAULT directly in
these newly-added helpers if CONFIG_ARM64_LSE_ATOMICS is disabled.

Signed-off-by: Hou Tao <redacted>

Hey Mark / Ard / Will / Catalin or others, could we get an Ack on patch 1 & 2
at min if it looks good to you?

I checked the instruction encodings in patches 1 and 2 and they all look
fine to me. However, after applying those two locally I get a build failure:

  | In file included from arch/arm64/net/bpf_jit_comp.c:23:
  | arch/arm64/net/bpf_jit_comp.c: In function ‘build_insn’:
  | arch/arm64/net/bpf_jit.h:94:2: error: implicit declaration of function ‘aarch64_insn_gen_stadd’; did you mean ‘aarch64_insn_gen_adr’? [-Werror=implicit-function-declaration]
  |    94 |  aarch64_insn_gen_stadd(Rn, Rs, A64_SIZE(sf))
  |       |  ^~~~~~~~~~~~~~~~~~~~~~
  | arch/arm64/net/bpf_jit_comp.c:912:9: note: in expansion of macro ‘A64_STADD’
  |   912 |    emit(A64_STADD(isdw, reg, src), ctx);
  |       |         ^~~~~~~~~
  | cc1: some warnings being treated as errors

Thanks for your review. The build failure is my fault. I update A64_STADD() in
patch 3 instead of patch 2
after replacing aarch64_insn_get_stadd() by aarch64_insn_gen_atomic_ld_op(), and
will fix it in v4. If you
are trying to test the encoder, I suggest you to apply patch 1~3.

Regards,
Tao

Will
.