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Date:   Wed, 2 Jun 2021 18:26:04 +0100
From:   Will Deacon <will@kernel.org>
To:     "Xu, Yanfei" <yanfei.xu@windriver.com>
Cc:     Daniel Borkmann <daniel@iogearbox.net>, ast@kernel.org,
        zlim.lnx@gmail.com, catalin.marinas@arm.com, andrii@kernel.org,
        kafai@fb.com, songliubraving@fb.com, yhs@fb.com,
        john.fastabend@gmail.com, kpsingh@kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] bpf: avoid unnecessary IPI in bpf_flush_icache
Message-ID: <20210602172603.GB31957@willie-the-truck>
References: <20210601150625.37419-1-yanfei.xu@windriver.com>
 <20210601150625.37419-2-yanfei.xu@windriver.com>
 <56cc1e25-25c3-a3da-64e3-8a1c539d685b@iogearbox.net>
 <20210601174114.GA29130@willie-the-truck>
 <7637dcdf-12b4-2861-3c76-f8a8e240a05e@windriver.com>
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On Wed, Jun 02, 2021 at 07:26:03PM +0800, Xu, Yanfei wrote:
> 
> 
> On 6/2/21 1:41 AM, Will Deacon wrote:
> > [Please note: This e-mail is from an EXTERNAL e-mail address]
> > 
> > On Tue, Jun 01, 2021 at 07:20:04PM +0200, Daniel Borkmann wrote:
> > > On 6/1/21 5:06 PM, Yanfei Xu wrote:
> > > > It's no need to trigger IPI for keeping pipeline fresh in bpf case.
> > > 
> > > This needs a more concrete explanation/analysis on "why it is safe" to do so
> > > rather than just saying that it is not needed.
> > 
> > Agreed. You need to show how the executing thread ends up going through a
> > context synchronizing operation before jumping to the generated code if
> > the IPI here is removed.
> 
> This patch came out with I looked through ftrace codes. Ftrace modify
> the text code and don't send IPI in aarch64_insn_patch_text_nosync(). I
> mistakenly thought the bpf is same with ftrace.
> 
> But now I'm still not sure why the ftrace don't need the IPI to go
> through context synchronizing, maybe the worst situation is omit a
> tracing event?

I think ftrace handles this itself via ftrace_sync_ipi, no?

Will