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

[Arnd Bergmann]

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.

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?

         Arnd