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

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Date:   Wed, 27 Mar 2019 10:17:18 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiong Wang <jiong.wang@...ronome.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 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?