Date:   Tue, 26 Mar 2019 19:47:58 +0000
From:   Jiong Wang <jiong.wang@...ronome.com>
To:     Edward Cree <ecree@...arflare.com>
Cc:     alexei.starovoitov@...il.com, daniel@...earbox.net,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        oss-drivers@...ronome.com
Subject: Re: [PATCH/RFC bpf-next 04/16] bpf: mark sub-register writes that
 really need zero extension to high bits

On 26/03/2019 18:44, Edward Cree wrote:
> On 26/03/2019 18:05, Jiong Wang wrote:
>> eBPF ISA specification requires high 32-bit cleared when low 32-bit
>> sub-register is written. This applies to destination register of ALU32 etc.
>> JIT back-ends must guarantee this semantic when doing code-gen.
>>
>> x86-64 and arm64 ISA has the same semantic, so the corresponding JIT
>> back-end doesn't need to do extra work. However, 32-bit arches (arm, nfp
>> etc.) and some other 64-bit arches (powerpc, sparc etc), need explicit zero
>> extension sequence to meet such semantic.
>>
>> This is important, because for code the following:
>>
>>    u64_value = (u64) u32_value
>>    ... other uses of u64_value
>>
>> compiler could exploit the semantic described above and save those zero
>> extensions for extending u32_value to u64_value. Hardware, runtime, or BPF
>> JIT back-ends, are responsible for guaranteeing this. Some benchmarks show
>> ~40% sub-register writes out of total insns, meaning ~40% extra code-gen (
>> could go up to more for some arches which requires two shifts for zero
>> extension) because JIT back-end needs to do extra code-gen for all such
>> instructions.
>>
>> However this is not always necessary in case u32_value is never cast into
>> a u64, which is quite normal in real life program. So, it would be really
>> good if we could identify those places where such type cast happened, and
>> only do zero extensions for them, not for the others. This could save a lot
>> of BPF code-gen.
>>
>> Algo:
>>   - Record indices of instructions that do sub-register def (write). And
>>     these indices need to stay with function state so path pruning and bpf
>>     to bpf function call could be handled properly.
>>
>>     These indices are kept up to date while doing insn walk.
>>
>>   - A full register read on an active sub-register def marks the def insn as
>>     needing zero extension on dst register.
>>
>>   - A new sub-register write overrides the old one.
>>
>>     A new full register write makes the register free of zero extension on
>>     dst register.
>>
>>   - When propagating register read64 during path pruning, it also marks def
>>     insns whose defs are hanging active sub-register, if there is any read64
>>     from shown from the equal state.
>>
>> Reviewed-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
>> Signed-off-by: Jiong Wang <jiong.wang@...ronome.com>
>> ---
>>   include/linux/bpf_verifier.h |  4 +++
>>   kernel/bpf/verifier.c        | 85 +++++++++++++++++++++++++++++++++++++++++---
>>   2 files changed, 84 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
>> index 27761ab..0ae9a3f 100644
>> --- a/include/linux/bpf_verifier.h
>> +++ b/include/linux/bpf_verifier.h
>> @@ -181,6 +181,9 @@ struct bpf_func_state {
>>   	 */
>>   	u32 subprogno;
>>   
>> +	/* tracks subreg definition. */
> Ideally this comment should mention that the stored value is the insn_idx
>   of the writing insn.  Perhaps also that this is safe because patching
>   (bpf_patch_insn_data()) only happens after main verification completes.

OK, will add more comments for this.

Regards,
Jiong

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