Re: [PATCH 1/5] riscv: nommu: remove PAGE_OFFSET hardcoding

[Jisheng Zhang <jszhang@kernel.org>]

On Mon, Mar 25, 2024 at 03:46:01PM -0700, Bo Gan wrote:
> On 3/25/24 9:40 AM, Jisheng Zhang wrote:
> > Currently, PAGE_OFFSET is hardcoded as 0x8000_0000, it works fine since
> > there's only one nommu platform in the mainline. However, there are
> > many cases where the (S)DRAM base address isn't 0x8000_0000, so remove
> > the hardcoding value, and introduce DRAM_BASE which will be set by
> > users during configuring. DRAM_BASE is 0x8000_0000 by default.
> > 
> > Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> > ---
> >   arch/riscv/Kconfig | 8 +++++++-
> >   1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 7895c77545f1..afd51dbdc253 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -247,10 +247,16 @@ config MMU
> >   	  Select if you want MMU-based virtualised addressing space
> >   	  support by paged memory management. If unsure, say 'Y'.
> > +if !MMU
> > +config DRAM_BASE
> > +	hex '(S)DRAM Base Address'
> > +	default 0x80000000
> > +endif
> > +
> >   config PAGE_OFFSET
> >   	hex
> >   	default 0xC0000000 if 32BIT && MMU
> > -	default 0x80000000 if !MMU
> > +	default DRAM_BASE if !MMU
> >   	default 0xff60000000000000 if 64BIT
> >   config KASAN_SHADOW_OFFSET
> > 
> 
> Thanks for this patch. I did something similar in my local nommu
> linux-6.8 tree in order to run it on the S7 hart of JH7110. I have
> another suggestion for you. Perhaps we should also make TASK_SIZE
> configurable, and let it default to `0xffffffff if 32BIT && !MMU`
> and `DRAM_BASE + DRAM_SIZE if 64BIT && !MMU`. Currently TASK_SIZE
> is effectively `0xffffffff if !MMU`, which doesn't work if I run
> rv64 linux-nommu with DDR that spans across 4G boundary.

I must admit that there's such nommu linux with 4GB DDR case in
theory, but it doesn't exist in real world: who will make such
strange platform ;) But anyway this improvement can be made when
the patchset talking about TASK_SIZE_MAX is settled down.

> 
> I see there's another patchset that tries to define TASK_SIZE_MAX
> for __access_ok(). Looks like that only affects the MMU case, and
> NOMMU is not touched. My aforementioned change won't conflict with
> it should it get merged.
> 
> Bo