Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20221124094845.1907443-1-debug@rivosinc.com> <20230720001852.GA572618@google.com>
 <CAJF2gTRFvSvEvQeDugdp73o7w4ArdtQ99JScEbLkaLnFcftVcA@mail.gmail.com>
In-Reply-To: <CAJF2gTRFvSvEvQeDugdp73o7w4ArdtQ99JScEbLkaLnFcftVcA@mail.gmail.com>
From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Mon, 24 Jul 2023 09:34:04 -0700
Message-ID: <CABCJKueNhc8qCbZbHJqdCB+PHHy0u5ETP4uWfpWBRaOMX6U6hA@mail.gmail.com>
Subject: Re: [PATCH v2] riscv: VMAP_STACK overflow detection thread-safe
To:     Guo Ren <guoren@kernel.org>
Cc:     Deepak Gupta <debug@rivosinc.com>, palmer@dabbelt.com,
        linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
        Jisheng Zhang <jszhang@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

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@googl=
e.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
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