linux-kernel - Re: [PATCH 00/13] riscv: compat: Add COMPAT mode support for rv64

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Date:   Wed, 22 Dec 2021 14:29:14 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Guo Ren <guoren@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Palmer Dabbelt <palmer@...belt.com>,
        Anup Patel <anup.patel@....com>,
        gregkh <gregkh@...uxfoundation.org>,
        liush <liush@...winnertech.com>, Wei Fu <wefu@...hat.com>,
        Drew Fustini <drew@...gleboard.org>,
        Wang Junqiang <wangjunqiang@...as.ac.cn>,
        Wei Wu (吴伟) <lazyparser@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-csky@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH 00/13] riscv: compat: Add COMPAT mode support for rv64

On Wed, Dec 22, 2021 at 1:59 PM Guo Ren <guoren@...nel.org> wrote:
> On Wed, Dec 22, 2021 at 2:10 AM Arnd Bergmann <arnd@...db.de> wrote:
> > On Tue, Dec 21, 2021 at 5:35 PM <guoren@...nel.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