Re: [PATCH bpf-next v2 4/9] bpf: Introduce load-acquire and store-release instructions

[Peilin Ye <yepeilin@google.com>]

Hi Alexei,

On Sat, Feb 08, 2025 at 01:30:46PM -0800, Alexei Starovoitov wrote:
> > Introduce BPF instructions with load-acquire and store-release
> > semantics, as discussed in [1].  The following new flags are defined:
> >
> >   BPF_ATOMIC_LOAD         0x10
> >   BPF_ATOMIC_STORE        0x20
> >   BPF_ATOMIC_TYPE(imm)    ((imm) & 0xf0)
> >
> >   BPF_RELAXED        0x0
> >   BPF_ACQUIRE        0x1
> >   BPF_RELEASE        0x2
> >   BPF_ACQ_REL        0x3
> >   BPF_SEQ_CST        0x4
> 
> I still don't like this.
> 
> Earlier you said:
> 
> > If yes, I think we either:
> >
> >  (a) add more flags to imm<4-7>: maybe LOAD_SEQ_CST (0x3) and
> >      STORE_SEQ_CST (0x6); need to skip OR (0x4) and AND (0x5) used by
> >      RMW atomics
> >  (b) specify memorder in imm<0-3>
> >
> > I chose (b) for fewer "What would be a good numerical value so that RMW
> > atomics won't need to use it in imm<4-7>?" questions to answer.
> >
> > If we're having dedicated fields for memorder, I think it's better to
> > define all possible values once and for all, just so that e.g. 0x2 will
> > always mean RELEASE in a memorder field.  Initially I defined all six of
> > them [2], then Yonghong suggested dropping CONSUME [3].
> 
> I don't think we should be defining "all possible values",
> since these are the values that llvm and C model supports,
> but do we have any plans to support anything bug ld_acq/st_rel ?
> I haven't heard anything.
> What even the meaning of BPF_ATOMIC_LOAD | BPF_ACQ_REL ?
> 
> What does the verifier suppose to do? reject for now? and then what?
> Map to what insn?
> 
> These values might imply that bpf infra is supposed to map all the values
> to cpu instructions, but that's not what we're doing here.
> We're only dealing with two specific instructions.
> We're not defining a memory model for all future new instructions.

Got it!  In v3, I'll change it back to:

  #define BPF_LOAD_ACQ   0x10
  #define BPF_STORE_REL  0x20

Thanks,
Peilin Ye