linux-kernel - Re: [PATCH bpf-next v4 0/3] bpf, arm64: use BPF prog pack allocator in BPF JIT

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Date:   Thu, 3 Aug 2023 18:32:40 +0200
From:   Florent Revest <revest@...omium.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Puranjay Mohan <puranjay12@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        bpf <bpf@...r.kernel.org>, KP Singh <kpsingh@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v4 0/3] bpf, arm64: use BPF prog pack allocator
 in BPF JIT

On Thu, Aug 3, 2023 at 6:16 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Aug 3, 2023 at 4:13 AM Mark Rutland <mark.rutland@....com> wrote:
> >
> > Hi Alexei,
> >
> > On Wed, Aug 02, 2023 at 02:02:39PM -0700, Alexei Starovoitov wrote:
> > > Mark, Catalin, Florent, KP,

Maybe you've missed my Acked-by for the series Alexei ?
https://lore.kernel.org/all/CABRcYmLAzhG=o2wcBNBtFP34Aj3+eYsEMtMREDT7SqNzBc9-qw@mail.gmail.com/

> > > This patch set was submitted on June 26 !
> >
> > I appreciate this was sent a while ago, but I have been stuck on some urgent
> > bug-fixing for the last few weeks, and my review bandwidth is therfore very
> > limited.
> >
> > Given Puranjay had previously told me he was doing this as a side project for
> > fun, and given no-one had told me this was urgent, I assumed that this wasn't a
> > major blocker and could wait.
> >
> > I should have sent a holding reply to that effect; sorry.
> >
> > The series addresses my original concern. However, in looking at it I think
> > there may me a wider potential isssue w.r.t. the way instruction memory gets
> > reused, because as writtten today the architecture doesn't seem to have a
> > guarantee on when instruction fetches are completed and therefore when it's
> > safe to modify instruction memory. Usually we're saved by TLB maintenance,
> > which this series avoids by design.

But I must say that this sits firmly outside of my knowledge of the
arm architectural details and I would totally miss this sort of nuance
so this is best handled by arm64 maintainers :)

> > I unfortunately haven't had the time to dig into that, poke our architects,
> > etc.
> >
> > So how urgent is this?
>
> The performance wins are substantial.
> We'd like to realize them sooner than later.

I've worked with Mark before, I know for a fact that he is dragged in
all directions. Until we figure out a way to clone him we should try
to not burn him out too often... :)