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

[Samuel Holland <samuel.holland@sifive.com>]

On 2024-03-25 8:28 PM, Jisheng Zhang wrote:
> 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

Another option would be to change this to:

  hex "DRAM Base Address" if !MMU

so the prompt is only visible for NOMMU, but we don't need a new symbol.

>>>   	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.

This case is quite easy to hit with QEMU :) In fact I sent a patch making this
exact change:

https://lore.kernel.org/linux-riscv/20240227003630.3634533-2-samuel.holland@sifive.com/

It's not really related to TASK_SIZE_MAX. access_ok() is a no-op on NOMMU,
because you can't prevent userspace from poking the kernel anyway.

Regards,
Samuel