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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 22 Dec 2021 14:29:14 +0100
Message-ID: <CAK8P3a07-v8WXxD7SHS0j42=bZgqDrOKGmisiuz=DbvYno5-SQ@mail.gmail.com>
Subject: Re: [PATCH 00/13] riscv: compat: Add COMPAT mode support for rv64
To:     Guo Ren <guoren@kernel.org>
Cc:     Arnd Bergmann <arnd@arndb.de>, Palmer Dabbelt <palmer@dabbelt.com>,
        Anup Patel <anup.patel@wdc.com>,
        gregkh <gregkh@linuxfoundation.org>,
        liush <liush@allwinnertech.com>, Wei Fu <wefu@redhat.com>,
        Drew Fustini <drew@beagleboard.org>,
        Wang Junqiang <wangjunqiang@iscas.ac.cn>,
        =?UTF-8?B?V2VpIFd1ICjlkLTkvJ8p?= <lazyparser@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        linux-csky@vger.kernel.org, Guo Ren <guoren@linux.alibaba.com>
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On Wed, Dec 22, 2021 at 1:59 PM Guo Ren <guoren@kernel.org> wrote:
> On Wed, Dec 22, 2021 at 2:10 AM Arnd Bergmann <arnd@arndb.de> wrote:
> > On Tue, Dec 21, 2021 at 5:35 PM <guoren@kernel.org> wrote:
>
> > There is still some benefit in b), but it would need to be weighed
> > against the downsides above. Can you explain in more detail what
> > use cases you have in mind, and which CPU cores actually support
> > this mode?
> The most reason is about b), see our customer's product:
> https://www.cnx-software.com/2021/10/25/allwinner-d1s-f133-risc-v-processor-64mb-ddr2/
>
> So I think all our next generation rv64 cores should support
> compat-mode. Compare to releasing rv32-full core, rv64 compat-mode is
> very cheap for our CPU design.
>
> You would get the answer when our new generation CPU is announced and it's soon.
>
> Currently, only qemu supports rv64 compact mode, that is my colleague
> (LIU Zhi Wei) contributed.

Right, that does make a lot of sense. I'm not sure we'll see a lot more of 64MB
DDR2 SiP implementations when a 128MB or even 256MB DDR3 configuration
has almost the same cost, but for any of those sizes I can see why you'd want to
run 32-bit user space, as well as 64-bit kernels.

         Arnd