[PATCH 00/13] riscv: compat: Add COMPAT mode support for rv64

[Guo Ren]

On Wed, Dec 22, 2021 at 2:10 AM Arnd Bergmann <arnd at arndb.de> wrote:
>
> On Tue, Dec 21, 2021 at 5:35 PM <guoren at kernel.org> wrote:
> >
> > From: Guo Ren <guoren at linux.alibaba.com>
> >
> > Currently, most 64-bit architectures (x86, parisc, powerpc, arm64,
> > s390, mips, sparc) have supported COMPAT mode. But they all have
> > history issues and can't use standard linux unistd.h. RISC-V would
> > be first standard __SYSCALL_COMPAT user of include/uapi/asm-generic
> > /unistd.h.
> >
> > The patchset are based on v5.16-rc6, you can compare rv64-compat32
> > v.s. rv32-whole in qemu with following step:
>
> Looks good overall, see my individual replies for minor comments I had.
Thx for the review :)

>
> I think there is a bigger question to answer though, which is whether this is
> actually a useful feature for rv64. In general, there are two reasons for
> wanting compat mode:
>
> a) compatibility with existing binaries and distros
>
> b) reducing the memory footprint of user space in a memory constrained
> environment, either deeply embedded or in a container.
>
> For the other architectures, a) is clearly the main driver, but equally so
> this is not the case on riscv, which does not have any legacy 32-bit
> code. Without that, adding compat mode would mainly introduce a
> second ABI to a lot of environments that at the moment only need to
> support one, and that adds complexity to the implementation and
> the extra attack surface of the second syscall ABI when an exploit
> may be possible only in compat mode.
>
> 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.

>
>          Arnd



-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/