Re: [PATCH v3 4/4] RISC-V: Allow booting kernel from any 4KB aligned address

[Anup Patel]

On Thu, Mar 28, 2019 at 1:25 PM Mike Rapoport <rppt@xxxxxxxxxxxxx> wrote:
>
> On Wed, Mar 27, 2019 at 12:54:41AM -0700, Christoph Hellwig wrote:
> > On Mon, Mar 25, 2019 at 09:46:59PM +0530, Anup Patel wrote:
> > > > Why do you even care about kernel mappings for non-existant ram.
> > >
> > > We care because there will always be some buggy kernel driver/code going
> > > out-of-bound and accessing non-existent RAM. If we by default map all
> > > possible kernel virtual address then behaviour of buggy accesses will be
> > > unpredictable.
> > >
> > > Further, I think we should also make .text and .rodata sections of kernel
> > > as read-only. This will protect kernel code and rodata.
> >
> > All of that is useful at the final_setup_vm() time - but none of it
> > matters during early setup_vm where life is complicated.
> >
> > Mike suggested on the previous iteration that you only do smaller
> > mappings when setting up the final mapping to avoid the ops churn,
> > and I fully agree with him.
> >
> > So I would suggest we avoid complicated the fiddly early boot changes
> > that just add complxity, and you instead redirect your efforts to
> > say implemented proper ro and non-executable sections using 4k mappings
> > in the final VM setup only.  That should actuall lead to less code
> > and complexity, and provide more benefits.
>
> It might be worth keeping trampoline_pg_dir if we are to split setup_vm().
> Then setup_vm() will only initialize the trampoline_pg_dir and
> final_setup_vm() will setup the swapper_pg_dir and switch to it.
> Otherwise final_setup_vm() would need to update live mappings which might
> be fragile.
>

We finally know the purpose trampoline_pg_dir page table.

The trampoline_pg_dir is suppose to contain only one 2M/4M mapping
to handle case where PAGE_OFFSET < load_address.

For 64bit systems, the PAGE_OFFSET is very high value typically
0xFFFFxxxxxxxxxxxx compared to RAM start 0x80000000. It is very
unlikely that we will have enormous RAM ending somewhere
0xFFFFxxxxxxxxxxxx.

For 32bit systems, it is quite possible that bootloader loads kernel at
load_address > 0x80000000. Let say PAGE_OFFSET = 0xC0000000
and load_address = 0xC0100000. Now the instruction which enables
MMU will be 0xC0100xxx and after enabling it will try to fetch next
instruction 0xC0100xxx + 4 but 0xC0100000 maps to 0xC0200000
as load_address > PAGE_OFFSET hence we will see wrong instruction
after enabling MMU.

(Note: Above explanation was provided by Anthony)

I guess we will have to keep both trampoline_pg_dir and swapper_pg_dir
init in setup_vm() because trampoline_pg_dir will only contain just
one 2M/4M mapping.

Regards,
Anup