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From:   Guo Ren <guoren@kernel.org>
Date:   Fri, 28 Jul 2023 20:01:07 +0800
Message-ID: <CAJF2gTThgCp8-KzghR0cFqDWXxZ2byLtLVF91GdRvBid7U+_aA@mail.gmail.com>
Subject: Re: [PATCH v2] riscv: VMAP_STACK overflow detection thread-safe
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Deepak Gupta <debug@rivosinc.com>, palmer@dabbelt.com,
        linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
        Jisheng Zhang <jszhang@kernel.org>
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On Tue, Jul 25, 2023 at 12:34=E2=80=AFAM Sami Tolvanen <samitolvanen@google=
.com> wrote:
>
> On Thu, Jul 20, 2023 at 8:06=E2=80=AFAM Guo Ren <guoren@kernel.org> wrote=
:
> >
> > On Thu, Jul 20, 2023 at 8:19=E2=80=AFAM Sami Tolvanen <samitolvanen@goo=
gle.com> wrote:
> > >
> > > Are you planning on resending this patch? I see it didn't gain much
> > > traction last time, but this looks like a much cleaner solution for
> > > selecting the overflow stack than having a `shadow_stack` and calling
> > > to C to compute the per-CPU offset. The asm_per_cpu macro also would
> > > come in handy when implementing CONFIG_SHADOW_CALL_STACK, which we'd
> > > like to have on RISC-V too.
> > I remember we ended up with an atomic lock mechanism instead of percpu
> > offset, so what's the benefit of percpu style in overflow_stack path?
>
> The benefit is not needing a separate temporary stack and locks just
Oh, you convinced me it could save another 1KB of memory.

Acked-by: Guo Ren <guoren@kernel.org>

> to compute the per-CPU offset. With CONFIG_SHADOW_CALL_STACK, we would
> also need a "shadow" shadow call stack in this case before calling to
> C code, at which point computing the offsets directly in assembly is
> just significantly cleaner and without concurrency issues.
>
> Sami


--=20
Best Regards
 Guo Ren