netdev - Re: [PATCH 1/1] bpf: avoid unnecessary IPI in bpf_flush

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Date:   Wed, 2 Jun 2021 18:26:04 +0100
From:   Will Deacon <will@...nel.org>
To:     "Xu, Yanfei" <yanfei.xu@...driver.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>, ast@...nel.org,
        zlim.lnx@...il.com, catalin.marinas@....com, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] bpf: avoid unnecessary IPI in bpf_flush_icache

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