Re: [PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch_]sub
From: Yonghong Song
Date: Mon Nov 30 2020 - 12:19:14 EST
On 11/28/20 5:34 PM, Alexei Starovoitov wrote:
On Fri, Nov 27, 2020 at 09:35:07PM -0800, Yonghong Song wrote:
On 11/27/20 9:57 AM, Brendan Jackman wrote:
Including only interpreter and x86 JIT support.
x86 doesn't provide an atomic exchange-and-subtract instruction that
could be used for BPF_SUB | BPF_FETCH, however we can just emit a NEG
followed by an XADD to get the same effect.
Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
---
arch/x86/net/bpf_jit_comp.c | 16 ++++++++++++++--
include/linux/filter.h | 20 ++++++++++++++++++++
kernel/bpf/core.c | 1 +
kernel/bpf/disasm.c | 16 ++++++++++++----
kernel/bpf/verifier.c | 2 ++
tools/include/linux/filter.h | 20 ++++++++++++++++++++
6 files changed, 69 insertions(+), 6 deletions(-)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 7431b2937157..a8a9fab13fcf 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -823,6 +823,7 @@ static int emit_atomic(u8 **pprog, u8 atomic_op,
/* emit opcode */
switch (atomic_op) {
+ case BPF_SUB:
case BPF_ADD:
/* lock *(u32/u64*)(dst_reg + off) <op>= src_reg */
EMIT1(simple_alu_opcodes[atomic_op]);
@@ -1306,8 +1307,19 @@ st: if (is_imm8(insn->off))
case BPF_STX | BPF_ATOMIC | BPF_W:
case BPF_STX | BPF_ATOMIC | BPF_DW:
- err = emit_atomic(&prog, insn->imm, dst_reg, src_reg,
- insn->off, BPF_SIZE(insn->code));
+ if (insn->imm == (BPF_SUB | BPF_FETCH)) {
+ /*
+ * x86 doesn't have an XSUB insn, so we negate
+ * and XADD instead.
+ */
+ emit_neg(&prog, src_reg, BPF_SIZE(insn->code) == BPF_DW);
+ err = emit_atomic(&prog, BPF_ADD | BPF_FETCH,
+ dst_reg, src_reg, insn->off,
+ BPF_SIZE(insn->code));
+ } else {
+ err = emit_atomic(&prog, insn->imm, dst_reg, src_reg,
+ insn->off, BPF_SIZE(insn->code));
+ }
if (err)
return err;
break;
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 6186280715ed..a20a3a536bf5 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -280,6 +280,26 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
.off = OFF, \
.imm = BPF_ADD | BPF_FETCH })
+/* Atomic memory sub, *(uint *)(dst_reg + off16) -= src_reg */
+
+#define BPF_ATOMIC_SUB(SIZE, DST, SRC, OFF) \
+ ((struct bpf_insn) { \
+ .code = BPF_STX | BPF_SIZE(SIZE) | BPF_ATOMIC, \
+ .dst_reg = DST, \
+ .src_reg = SRC, \
+ .off = OFF, \
+ .imm = BPF_SUB })
Currently, llvm does not support XSUB, should we support it in llvm?
At source code, as implemented in JIT, user can just do a negate
followed by xadd.
I forgot we have BPF_NEG insn :)
Indeed it's probably easier to handle atomic_fetch_sub() builtin
completely on llvm side. It can generate bpf_neg followed by atomic_fetch_add.
Just tried. llvm selectiondag won't be able to automatically
convert atomic_fetch_sub to neg + atomic_fetch_add. So there
will be a need in BPFInstrInfo.td to match atomic_fetch_sub IR
pattern. I will experiment this together with xsub.
No need to burden verifier, interpreter and JITs with it.