Re: [PATCH 2/3] Documentation: riscv: Add documentation that describes the VM layout

[Alex Ghiti <alex@ghiti.fr>]

Hi Arnd,

Le 3/11/21 à 3:42 AM, Arnd Bergmann a écrit :
> On Wed, Mar 10, 2021 at 8:12 PM Alex Ghiti <alex@ghiti.fr> wrote:
>> Le 3/10/21 à 6:42 AM, Arnd Bergmann a écrit :
>>> On Thu, Feb 25, 2021 at 12:56 PM Alex Ghiti <alex@ghiti.fr> wrote:
>>>>
>>>> Le 2/25/21 à 5:34 AM, David Hildenbrand a écrit :
>>>>>                     |            |                  |         |> +
>>>>> ffffffc000000000 | -256    GB | ffffffc7ffffffff |   32 GB | kasan
>>>>>> +   ffffffcefee00000 | -196    GB | ffffffcefeffffff |    2 MB | fixmap
>>>>>> +   ffffffceff000000 | -196    GB | ffffffceffffffff |   16 MB | PCI io
>>>>>> +   ffffffcf00000000 | -196    GB | ffffffcfffffffff |    4 GB | vmemmap
>>>>>> +   ffffffd000000000 | -192    GB | ffffffdfffffffff |   64 GB |
>>>>>> vmalloc/ioremap space
>>>>>> +   ffffffe000000000 | -128    GB | ffffffff7fffffff |  126 GB |
>>>>>> direct mapping of all physical memory
>>>>>
>>>>> ^ So you could never ever have more than 126 GB, correct?
>>>>>
>>>>> I assume that's nothing new.
>>>>>
>>>>
>>>> Before this patch, the limit was 128GB, so in my sense, there is nothing
>>>> new. If ever we want to increase that limit, we'll just have to lower
>>>> PAGE_OFFSET, there is still some unused virtual addresses after kasan
>>>> for example.
>>>
>>> Linus Walleij is looking into changing the arm32 code to have the kernel
>>> direct map inside of the vmalloc area, which would be another place
>>> that you could use here. It would be nice to not have too many different
>>> ways of doing this, but I'm not sure how hard it would be to rework your
>>> code, or if there are any downsides of doing this.
>>
>> This was what my previous version did: https://lkml.org/lkml/2020/6/7/28.
>>
>> This approach was not welcomed very well and it fixed only the problem
>> of the implementation of relocatable kernel. The second issue I'm trying
>> to resolve here is to support both 3 and 4 level page tables using the
>> same kernel without being relocatable (which would introduce performance
>> penalty). I can't do it when the kernel mapping is in the vmalloc region
>> since vmalloc region relies on PAGE_OFFSET which is different on both 3
>> and 4 level page table and that would then require the kernel to be
>> relocatable.
> 
> Ok, I see.
> 
> I suppose it might work if you moved the direct-map to the lowest
> address and the vmalloc area (incorporating the kernel mapping,
> modules, pio, and fixmap at fixed addresses) to the very top of the
> address space, but you probably already considered and rejected
> that for other reasons.
> 

Yes I considered it...when you re-proposed it :) I'm not opposed to your 
solution in the vmalloc region but I can't find any advantage over the 
current solution, are there ? That would harmonize with Linus's work, 
but then we'd be quite different from x86 address space.

And by the way, thanks for having suggested the current solution in a 
previous conversation :)

Thanks again,

Alex

>           Arnd
>