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

[Guo Ren]

On Sun, Dec 26, 2021 at 4:36 PM Jisheng Zhang <jszhang3 at mail.ustc.edu.cn> wrote:
>
> On Wed, 22 Dec 2021 20:59:30 +0800
> Guo Ren <guoren at kernel.org> wrote:
>
> > 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.
> >
>
> What about adding RV64 ILP32 support instead? This don't need HW side
> modifications so can benefit all RV64.
ILP32 is another topic in C Language Data Type Models and it couldn't
replace the standard rv32 ecosystem.
COMPAT is a common framework in Linux (7 arches have been supported),
so let rv64 support COMPAT mode is considerable.

Customers would choose ILP32 / RV32-compat by themself and that
depends on which one has a better ecosystem.

>
> Thanks
>
> > Currently, only qemu supports rv64 compact mode, that is my colleague
> > (LIU Zhi Wei) contributed.
>


-- 
Best Regards
 Guo Ren

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