Re: [PATCH 0/3] Highmem support for 32-bit RISC-V

[Arnd Bergmann]

On Tue, Apr 14, 2020 at 5:17 PM afzal mohammed <afzal.mohd.ma@xxxxxxxxx> wrote:
>
> + rmk
>
> Hi Arnd,
>
> On Wed, Apr 08, 2020 at 04:40:17PM +0200, Arnd Bergmann wrote:
>
> > No code yet, so far not much more than the ideas that I listed. We
> > are currently looking for someone interested in doing the work
> > or maybe sponsoring it if they have a strong interest.
>
> If no one have yet taken it up, i am interested in doing the work, i
> will sponsor myself :). i will proceed at a slow pace without derailing
> my other things normal.
>
> To keep expectations realistic: i have not yet taken task of this
> complexity, it is more of a learning for me. My familiarity with Linux
> at the ARM architecture level is mostly on no-MMU (Cortex-M), have not
> worked so far seriously on MMU Linux at the ARM architectural level,
> though used to go through ARM ARM v7-AR at times.

Thanks for offering to help, it's very much appreciated. Let me know how
it goes and if you have any more detailed questions.

> i have a few 32-bit ARM Cortex-A (A5, A8 & A9) boards, maximum RAM 1G,
> none have LPAE, seems i have to buy one for this purpose.

I would recommend starting in a qemu emulated system on a PC host,
you can just set it to emulate a Cortex-A15 or A7, and you can attach
gdb to the qemu instance to see where it crashes (which it inevitably
will).

You can also start by changing the functions in asm/uaccess.h to
use the linear kernel mapping and memcpy(), like the version in
arch/um/kernel/skas/uaccess.c does. This is slow, but will work on
regardless of whether user space is mapped, and you can do a
generic implementation that works on any architecture and put that
into include/asm-generic/uaccess.h.

A second step after that could be to unmap user space when entering
the kernel, without any change in the memory layout, this is still
mostly hardware independent and could easily be done in qemu
or any 32-bit ARM CPU.

Another thing to try early is to move the vmlinux virtual address
from the linear mapping into vmalloc space. This does not require
LPAE either, but it only works on relatively modern platforms that
don't have conflicting fixed mappings there.

If you get that far, I'll happily buy you a Raspberry Pi 4 with 4GB
for further experiments ;-)
That one can run both 64-bit and 32-bit kernels (with LPAE),
so you'd be able to test the limits and not rely on qemu to find
all bugs such as missing TLB flushes or barriers.

        Arnd