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

[Alex Ghiti]

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@xxxxxxxx> 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@xxxxxxxx> 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