netdev - Re: [PATCH/RFC bpf-next 06/16] bpf: new sysctl "bpf_jit_32bit

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Date:   Wed, 27 Mar 2019 17:18:35 +0000
From:   Jiong Wang <jiong.wang@...ronome.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, oss-drivers@...ronome.com
Subject: Re: [PATCH/RFC bpf-next 06/16] bpf: new sysctl "bpf_jit_32bit_opt"


> On 27 Mar 2019, at 17:17, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> On Wed, Mar 27, 2019 at 05:06:01PM +0000, Jiong Wang wrote:
>> 
>>> On 27 Mar 2019, at 17:00, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
>>> 
>>> On Tue, Mar 26, 2019 at 06:05:29PM +0000, Jiong Wang wrote:
>>>> After previous patches, verifier has marked those instructions that really
>>>> need zero extension on dst_reg.
>>>> 
>>>> It is then for all back-ends to decide how to use such information to
>>>> eliminate unnecessary zero extension codegen during JIT compilation.
>>>> 
>>>> One approach is:
>>>> 1. Verifier insert explicit zero extension for those instructions that
>>>>    need zero extension.
>>>> 2. All JIT back-ends do NOT generate zero extension for sub-register
>>>>    write any more.
>>>> 
>>>> The good thing for this approach is no major change on JIT back-end
>>>> interface, all back-ends could get this optimization.
>>>> 
>>>> However, only those back-ends that do not have hardware zero extension
>>>> want this optimization. For back-ends like x86_64 and AArch64, there is
>>>> hardware support, so this optimization should be disabled.
>>>> 
>>>> This patch introduces new sysctl "bpf_jit_32bit_opt" which is the control
>>>> variable for whether the optimization should be enabled.
>>>> 
>>>> It is initialized using target hook bpf_jit_hardware_zext which is default
>>>> true, meaning the underlying hardware will do zero extension automatically,
>>>> therefore the optimization will be disabled.
>>>> 
>>>> Offload targets do not use this native target hook, instead, they could
>>>> get the optimization results using bpf_prog_offload_ops.finalize.
>>>> 
>>>> The user could always enable or disable the optimization by using:
>>>> 
>>>>  sysctl net/core/bpf_jit_32bit_opt=[0 | 1]
>>> 
>>> I don't think there should be a sysctl for this.
>> 
>> The sysctl introduced mostly because I think it could be useful for testing.
>> For example on x86_64, with this sysctl, we can enable the optimisation and
>> can run selftest.
>> 
>> Does this make sense?
>> 
>> Or when one insn is marked, we print verbose info, so the tester could catch
>> it from log?
> 
> sysctl in this patch only triggers insertion of shifts.
> what kind of testing does it enable on x64?
> The writing insn is already 32-bit and hw does zero extend.
> These two shifts is always a nop?
> a sysctl to test that the verifier inserted shifts in the right place?

Yes, that’s the test methodology I am using. Match the instruction sequence after
shifts insertion.

Regards,
Jiong