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

[Peilin Ye]

On Sat, Feb 08, 2025 at 07:46:54PM -0800, Alexei Starovoitov wrote:
> > > 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
> 
> why not 1 and 2 ?
> All other bits are reserved and the verifier will make sure they're zero,
> so when/if we need to extend it then it wouldn't matter whether
> lower 4 bits are reserved or other bits.
> Say, we decide to support cmpwait_relaxed as a new insn.
> It can take the value 3 and arm64 JIT will map it to ldxr+wfe+...
> 
> Then with this new load_acq and cmpwait_relaxed we can efficiently
> implement both smp_cond_load_relaxed and smp_cond_load_acquire.

Ah, I see.  When you suggested "LOAD_ACQ=1 and STORE_REL=2" earlier, I
didn't realize you meant 1 and 2 in imm<0-3>.

Thanks,
Peilin Ye