Subject: Re: [PATCH v6 bpf-next 04/17] bpf: introduce new alu insn BPF_ZEXT for explicit zero extension

From: Jiong Wang <[email-masked]>
Date: Tue, 07 May 2019 05:29:09 +0100

Jiong Wang writes:

> Alexei Starovoitov writes:
>
>> On Fri, May 03, 2019 at 11:42:31AM +0100, Jiong Wang wrote:
>>> This patch introduce new alu32 insn BPF_ZEXT, and allocate the unused
>>> opcode 0xe0 to it.
>>> 
>>> Compared with the other alu32 insns, zero extension on low 32-bit is the
>>> only semantics for this instruction. It also allows various JIT back-ends
>>> to do optimal zero extension code-gen.
>>> 
>>> BPF_ZEXT is supposed to be encoded with BPF_ALU only, and is supposed to be
>>> generated by the latter 32-bit optimization code inside verifier for those
>>> arches that do not support hardware implicit zero extension only.
>>> 
>>> It is not supposed to be used in user's program directly at the moment.
>>> Therefore, no need to recognize it inside generic verification code. It
>>> just need to be supported for execution on interpreter or related JIT
>>> back-ends.
>>
>> uapi and the doc define it, but "it is not supposed to be used" ?!
>>
>>> Signed-off-by: Jiong Wang <[email-masked]>
>>> ---
>>>  Documentation/networking/filter.txt | 10 ++++++++++
>>>  include/uapi/linux/bpf.h            |  3 +++
>>>  kernel/bpf/core.c                   |  4 ++++
>>>  tools/include/uapi/linux/bpf.h      |  3 +++
>>>  4 files changed, 20 insertions(+)
>>> 
>>> diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt
>>> index 319e5e0..1cb3e42 100644
>>> --- a/Documentation/networking/filter.txt
>>> +++ b/Documentation/networking/filter.txt
>>> @@ -903,6 +903,16 @@ If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 [ in eBPF ], BPF_OP(code) is one of:
>>>    BPF_MOV   0xb0  /* eBPF only: mov reg to reg */
>>>    BPF_ARSH  0xc0  /* eBPF only: sign extending shift right */
>>>    BPF_END   0xd0  /* eBPF only: endianness conversion */
>>> +  BPF_ZEXT  0xe0  /* eBPF BPF_ALU only: zero-extends low 32-bit */
>>> +
>>> +Compared with BPF_ALU | BPF_MOV which zero-extends low 32-bit implicitly,
>>> +BPF_ALU | BPF_ZEXT zero-extends low 32-bit explicitly. Such zero extension is
>>
>> wait. that's an excellent observation. alu|mov is exactly it.
>> we do not need another insn.
>> we probably can teach the verifier to recognize <<32, >>32 and replace
>> with mov32
>
> Hmm, I am silly, in v6, patched insn will be conservatively marked as
> always needing zext, so looks like no problem to just insert mov32 as
> zext. But some backends needs minor opt, because this will be special mov,
> with the same src and dst, just need to clear high 32-bit, no need of
> mov.

I take it back.

Recalled the reason why new ZEXT was introduced. It was because instruction
insertion based approach doesn't push analysis results down to JIT
back-ends, instead, it removes the clear-high-32bit semantics from
all sub-register write instructions, then insert new explicit ZEXT insn,
either by 64bit shifts combination or this new introduced ZEXT, what's
important, the inserted "ZEXT" should not be affected by
"env->verifier_zext", and be performed unconditionally.

That is to say, for the current zero extension insertion based approach,
JIT back-ends trust verifier has done full zero extension insertion and
rewritten the instruction sequence once the flag env->verifier_zext is set,
and then JIT back-end do NOT clear high 32-bit for all existing
sub-register write instructions, for example ALU32 and narrowed load, they
rely on those new inserted "unconditional ZEXT" to do the job if it is
needed. So, if we insert mov32, there actually won't be zero extension
insertion performed for it.

The inserted "ZEXT" needs to have zero extension semantics that is not
affected by env->verifier_zext. BPF_ZEXT was introduced because of this,
low32 zext is its only semantics and should be unconditionally done.

"mov32" could be used as "ZEXT" only when there is no such removal of zero
extension semantics from alu32, or if JIT back-end could have instruction
level information, for example the analyzed instruction level zero extension
information pushed down to JIT back-ends. The new inserted "mov32" would
then has information like "zext_dst" be true, JIT back-end then will
generate zero extension for it.

Regards,
Jiong