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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 22 Jul 2020 22:22:25 +0200
Message-ID: <CAK8P3a2VHXDLK6iba=NxSQ-t=9P7LSwzwx3XrK=N=M+qoX_oeQ@mail.gmail.com>
Subject: Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone
To:     Palmer Dabbelt <palmer@dabbelt.com>
Cc:     Alexandre Ghiti <alex@ghiti.fr>, Albert Ou <aou@eecs.berkeley.edu>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Linux-MM <linux-mm@kvack.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anup Patel <Anup.Patel@wdc.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Atish Patra <Atish.Patra@wdc.com>,
        Paul Mackerras <paulus@samba.org>,
        Zong Li <zong.li@sifive.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
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On Wed, Jul 22, 2020 at 9:52 PM Palmer Dabbelt <palmer@dabbelt.com> wrote:
> On Wed, 22 Jul 2020 02:43:50 PDT (-0700), Arnd Bergmann wrote:
> > On Tue, Jul 21, 2020 at 9:06 PM Palmer Dabbelt <palmer@dabbelt.com> wrote:
> > The eventual goal is to have a split of 3840MB for either user or linear map
> > plus and 256MB for vmalloc, including the kernel. Switching between linear
> > and user has a noticeable runtime overhead, but it relaxes both the limits
> > for user memory and lowmem, and it provides a somewhat stronger
> > address space isolation.
>
> Ya, I think we decided not to do that, at least for now.  I guess the right
> answer there will depend on what 32-bit systems look like, and since we don't
> have any I'm inclined to just stick to the fast option.

Makes sense. Actually on 32-bit Arm we see fewer large-memory
configurations in new machines than we had in the past before 64-bit
machines were widely available at low cost, so I expect not to see a
lot new hardware with more than 1GB of DDR3 (two 256Mbit x16 chips)
for cost reasons, and rv32 is likely going to be similar, so you may never
really see a need for highmem or the above hack to increase the
size of the linear mapping.

I just noticed that rv32 allows 2GB of lowmem rather than just the usual
768MB or 1GB, at the expense of addressable user memory. This seems
like an unusual choice, but I also don't see any reason to change this
or make it more flexible unless actual users appear.

       Arnd