Re: [PATCH] bpf: optimize constant blinding

[Edward Cree <ecree@solarflare.com>]

On 17/06/2019 20:59, Jiong Wang wrote:
> Edward Cree writes:
>
>> On 14/06/2019 16:13, Jiong Wang wrote:
>>> Just an update and keep people posted.
>>>
>>> Working on linked list based approach, the implementation looks like the
>>> following, mostly a combine of discussions happened and Naveen's patch,
>>> please feel free to comment.
>>>
>>>   - Use the reserved opcode 0xf0 with BPF_ALU as new pseudo insn code
>>>     BPF_LIST_INSN. (0xf0 is also used with BPF_JMP class for tail call).
>>>
>>>   - Introduce patch pool into bpf_prog->aux to keep all patched insns.
>> It's not clear to me what the point of the patch pool is, rather than just
>>  doing the patch straight away. 
> I used pool because I was thinking insn to be patched could be high
> percentage, so doing lots of alloc call is going to be less efficient? so
> allocate a big pool, and each time when creating new patch node, allocate
> it from the pool directly. Node is addressed using pool_base + offset, each
> node only need to keep offset.
Good idea; but in that case it doesn't need to be a pool of patches (storing
 their prev and next), just a pool of insns.  I.e. struct bpf_insn pool[many];
 then in orig prog when patching an insn replace it with BPF_LIST_INSN.  If
 we later decide to patch an insn within a patch, we can replace it (i.e. the
 entry in bpf_insn_pool) with another BPF_LIST_INSN pointing to some later bit
 of the pool, then we just have a little bit of recursion at linearise time.
Does that work?

-Ed