On Wed, 22 Dec 2021 20:59:30 +0800 Guo Ren <guoren@xxxxxxxxxx> wrote: > On Wed, Dec 22, 2021 at 2:10 AM Arnd Bergmann <arnd@xxxxxxxx> wrote: > > > > On Tue, Dec 21, 2021 at 5:35 PM <guoren@xxxxxxxxxx> wrote: > > > > > > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx> > > > > > > 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. Thanks > Currently, only qemu supports rv64 compact mode, that is my colleague > (LIU Zhi Wei) contributed.
