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.
No need to burden verifier, interpreter and JITs with it.
