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

[afzal mohammed]

Hi,

On Tue, Apr 14, 2020 at 09:29:46PM +0200, Arnd Bergmann wrote:
> On Tue, Apr 14, 2020 at 5:17 PM afzal mohammed <afzal.mohd.ma@xxxxxxxxx> wrote:

> > + rmk

[ Forgot to provide context to Russell - this is about implementing
  VMSPLIT_4G_4G support for 32-bit ARM as a possible replacement of
  highmem ]

> > 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.

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

Okay, i will proceed initially w/ things that can be done using qemu &
available ARM boards. Right now no questions, i will probably be coming
up with questions later.

Regards
afzal

> 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.