[RFC PATCH 00/22] riscv: s64ilp32: Running 32-bit Linux kernel on 64-bit supervisor mode

[Guo Ren]

On Sat, May 20, 2023 at 12:54 AM Arnd Bergmann <arnd at arndb.de> wrote:
>
> On Fri, May 19, 2023, at 17:31, Guo Ren wrote:
> > On Fri, May 19, 2023 at 2:29 AM Arnd Bergmann <arnd at arndb.de> wrote:
> >> On Thu, May 18, 2023, at 17:38, Palmer Dabbelt wrote:
> >> > On Thu, 18 May 2023 06:09:51 PDT (-0700), guoren at kernel.org wrote:
> >>
> >> If for some crazy reason you'd still want the 64ilp32 ABI in user
> >> space, running the kernel this way is probably still a bad idea,
> >> but that one is less clear. There is clearly a small memory
> >> penalty of running a 64-bit kernel for larger data structures
> >> (page, inode, task_struct, ...) and vmlinux, and there is no
> > I don't think it's a small memory penalty, our measurement is about
> > 16% with defconfig, see "Why 32-bit Linux?" section.
> >
> > This patch series doesn't add 64ilp32 userspace abi, but it seems you
> > also don't like to run 32-bit Linux kernel on 64-bit hardware, right?
>
> Ok, I'm sorry for missing the important bit here. So if this can
> still use the normal 32-bit user space, the cost of this patch set
> is not huge, and it's something that can be beneficial in a few
> cases, though I suspect most users are still better off running
> 64-bit kernels.
>
> > The motivation of s64ilp32 (running 32-bit Linux kernel on 64-bit s-mode):
> >  - The target hardware (Canaan Kendryte k230) only supports MXL=64,
> > SXL=64, UXL=64/32.
> >  - The 64-bit Linux + compat 32-bit app can't satisfy the 64/128MB scenarios.
> >
> >> huge additional maintenance cost on top of the ABI itself
> >> that you'd need either way, but using a 64-bit address space
> >> in the kernel has some important advantages even when running
> >> 32-bit userland: processes can use the entire 4GB virtual
> >> space, while the kernel can address more than 768MB of lowmem,
> >> and KASLR has more bits to work with for randomization. On
> >> RISCV, some additional features (VMAP_STACK, KASAN, KFENCE,
> >> ...) depend on 64-bit kernels even though they don't
> >> strictly need that.
> >
> > I agree that the 64-bit linux kernel has more functionalities, but:
> >  - What do you think about linux on a 64/128MB SoC? Could it be
> > affordable to VMAP_STACK, KASAN, KFENCE?
>
> I would definitely recommend VMAP_STACK, but that can be implemented
> and is used on other 32-bit architectures (ppc32, arm32) without a
> huge cost. The larger virtual user address space can help even on
> machines with 128MB, though most applications probably don't care at
> that point.
Good point, I would support VMAP_STACK in ARCH_RV64ILP32.


>
> >  - I think 32-bit Linux & RTOS have monopolized this market (64/128MB
> > scenarios), right?
>
> The minimum amount of RAM that makes a system usable for Linux is
> constantly going up, so I think with 64MB, most new projects are
> already better off running some RTOS kernel instead of Linux.
> The ones that are still usable today probably won't last a lot
> of distro upgrades before the bloat catches up with them, but I
> can see how your patch set can give them a few extra years of
> updates.
Linux development costs much cheaper than RTOS, so the vendors would
first develop a Linux version. If it succeeds in the market, the
vendors will create a cost-down solution. So their first choice is to
cut down the memory footprint of the first Linux version instead of
moving to RTOS.

With the price of 128MB-DDR3 & 64MB-DDR2 being more and more similar,
32bit-Linux has more opportunities to instead of RTOS.

>
> For the 256MB+ systems, I would expect the sensitive kernel
> allocations to be small enough that the series makes little
> difference. The 128MB systems are the most interesting ones
> here, and I'm curious to see where you spot most of the
> memory usage differences, I'll also reply to your initial
> mail for that.
Thx, I aslo recommand you read about "Why s64ilp32 has better
performance?" section :)
How do you think running arm32-Linux on coretex-A35/A53/A55?

>
>        Arnd



-- 
Best Regards
 Guo Ren