Re: [PATCH v1 bpf-next 3/9] bpf: expose how xlated insns map to jitted insns

[Anton Protopopov <aspsk@isovalent.com>]

On Mon, Feb 05, 2024 at 05:09:51PM -0800, Alexei Starovoitov wrote:
> On Fri, Feb 2, 2024 at 8:34 AM Anton Protopopov <aspsk@isovalent.com> wrote:
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 4def3dde35f6..bdd6be718e82 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -1524,6 +1524,13 @@ struct bpf_prog_aux {
> >         };
> >         /* an array of original indexes for all xlated instructions */
> >         u32 *orig_idx;
> > +       /* for every xlated instruction point to all generated jited
> > +        * instructions, if allocated
> > +        */
> > +       struct {
> > +               u32 off;        /* local offset in the jitted code */
> > +               u32 len;        /* the total len of generated jit code */
> > +       } *xlated_to_jit;
> 
> Simply put Nack to this approach.
> 
> Patches 2 and 3 add an extreme amount of memory overhead.
> 
> As we discussed during office hours we need a "pointer to insn" concept
> aka "index on insn".
> The verifier would need to track that such things exist and adjust
> indices of insns when patching affects those indices.
> 
> For every static branch there will be one such "pointer to insn".
> Different algorithms can be used to keep them correct.
> The simplest 'lets iterate over all such pointers and update them'
> during patch_insn() may even be ok to start.
>
> Such "pointer to insn" won't add any memory overhead.
> When patch+jit is done all such "pointer to insn" are fixed value.

Ok, thanks for looking, this makes sense.