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Date:   Thu, 3 Aug 2023 12:13:00 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Puranjay Mohan <puranjay12@gmail.com>,
        Florent Revest <revest@chromium.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        bpf <bpf@vger.kernel.org>, KP Singh <kpsingh@kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf-next v4 0/3] bpf, arm64: use BPF prog pack allocator
 in BPF JIT
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Hi Alexei,

On Wed, Aug 02, 2023 at 02:02:39PM -0700, Alexei Starovoitov wrote:
> On Sun, Jul 30, 2023 at 10:22 AM Puranjay Mohan <puranjay12@gmail.com> wrote:
> >
> > Hi Mark,
> > I am really looking forward to your feedback on this series.
> >
> > On Mon, Jul 17, 2023 at 9:50 AM Puranjay Mohan <puranjay12@gmail.com> wrote:
> > >
> > > Hi Mark,
> > >
> > > On Mon, Jul 3, 2023 at 7:15 PM Mark Rutland <mark.rutland@arm.com> wrote:
> > > >
> > > > On Mon, Jul 03, 2023 at 06:40:21PM +0200, Daniel Borkmann wrote:
> > > > > Hi Mark,
> > > >
> > > > Hi Daniel,
> > > >
> > > > > On 6/26/23 10:58 AM, Puranjay Mohan wrote:
> > > > > > BPF programs currently consume a page each on ARM64. For systems with many BPF
> > > > > > programs, this adds significant pressure to instruction TLB. High iTLB pressure
> > > > > > usually causes slow down for the whole system.
> > > > > >
> > > > > > Song Liu introduced the BPF prog pack allocator[1] to mitigate the above issue.
> > > > > > It packs multiple BPF programs into a single huge page. It is currently only
> > > > > > enabled for the x86_64 BPF JIT.
> > > > > >
> > > > > > This patch series enables the BPF prog pack allocator for the ARM64 BPF JIT.
> > > >
> > > > > If you get a chance to take another look at the v4 changes from Puranjay and
> > > > > in case they look good to you reply with an Ack, that would be great.
> > > >
> > > > Sure -- this is on my queue of things to look at; it might just take me a few
> > > > days to get the time to give this a proper look.
> > > >
> > > > Thanks,
> > > > Mark.
> > >
> > > I am eagerly looking forward to your feedback on this series.
> 
> Mark, Catalin, Florent, KP,
> 
> 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.

I unfortunately haven't had the time to dig into that, poke our architects,
etc.

So how urgent is this?

Thanks,
Mark.