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

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Message-ID: <7637dcdf-12b4-2861-3c76-f8a8e240a05e@windriver.com>
Date:   Wed, 2 Jun 2021 19:26:03 +0800
From:   "Xu, Yanfei" <yanfei.xu@...driver.com>
To:     Will Deacon <will@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     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 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?

Thanks,
Yanfei

> 
> Will
>