| | | |> +
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.
--
Thanks,
David / dhildenb
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.
Thanks,
Alex
On Thu, Feb 25, 2021 at 12:56 PM Alex Ghiti <[email protected]> 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.
Arnd
Hi Arnd,
Le 3/10/21 à 6:42 AM, Arnd Bergmann a écrit :
> On Thu, Feb 25, 2021 at 12:56 PM Alex Ghiti <[email protected]> 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.
Alex
>
> Arnd
>
> _______________________________________________
> linux-riscv mailing list
> [email protected]
> http://lists.infradead.org/mailman/listinfo/linux-riscv
>
On Wed, Mar 10, 2021 at 8:12 PM Alex Ghiti <[email protected]> wrote:
> Le 3/10/21 à 6:42 AM, Arnd Bergmann a écrit :
> > On Thu, Feb 25, 2021 at 12:56 PM Alex Ghiti <[email protected]> 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.
Arnd
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 <[email protected]> wrote:
>> Le 3/10/21 à 6:42 AM, Arnd Bergmann a écrit :
>>> On Thu, Feb 25, 2021 at 12:56 PM Alex Ghiti <[email protected]> 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
>
On Sat, Mar 13, 2021 at 9:23 AM Alex Ghiti <[email protected]> wrote:
>
> 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 :)
Ah, I really need to keep track better of what I already commented on...
Arnd