[PATCH v2 2/4] arm64: mm: extend linear region for 52-bit VA configurations

[Ard Biesheuvel]

On Wed, 14 Oct 2020 at 05:45, Anshuman Khandual
<anshuman.khandual at arm.com> wrote:
>
>
>
> On 10/08/2020 09:06 PM, Ard Biesheuvel wrote:
> > For historical reasons, the arm64 kernel VA space is configured as two
> > equally sized halves, i.e., on a 48-bit VA build, the VA space is split
> > into a 47-bit vmalloc region and a 47-bit linear region.
> >
> > When support for 52-bit virtual addressing was added, this equal split
> > was kept, resulting in a substantial waste of virtual address space in
> > the linear region:
> >
> >                            48-bit VA                     52-bit VA
> >   0xffff_ffff_ffff_ffff +-------------+               +-------------+
> >                         |   vmalloc   |               |   vmalloc   |
> >   0xffff_8000_0000_0000 +-------------+ _PAGE_END(48) +-------------+
> >                         |   linear    |               :             :
> >   0xffff_0000_0000_0000 +-------------+               :             :
> >                         :             :               :             :
> >                         :             :               :             :
> >                         :             :               :             :
> >                         :             :               :  currently  :
> >                         :  unusable   :               :             :
> >                         :             :               :   unused    :
> >                         :     by      :               :             :
> >                         :             :               :             :
> >                         :  hardware   :               :             :
> >                         :             :               :             :
> >   0xfff8_0000_0000_0000 :             : _PAGE_END(52) +-------------+
> >                         :             :               |             |
> >                         :             :               |             |
> >                         :             :               |             |
> >                         :             :               |             |
> >                         :             :               |             |
> >                         :  unusable   :               |             |
> >                         :             :               |   linear    |
> >                         :     by      :               |             |
> >                         :             :               |   region    |
> >                         :  hardware   :               |             |
> >                         :             :               |             |
> >                         :             :               |             |
> >                         :             :               |             |
> >                         :             :               |             |
> >                         :             :               |             |
> >                         :             :               |             |
> >   0xfff0_0000_0000_0000 +-------------+  PAGE_OFFSET  +-------------+
> >
> > As illustrated above, the 52-bit VA kernel uses 47 bits for the vmalloc
> > space (as before), to ensure that a single 64k granule kernel image can
> > support any 64k granule capable system, regardless of whether it supports
> > the 52-bit virtual addressing extension. However, due to the fact that
> > the VA space is still split in equal halves, the linear region is only
> > 2^51 bytes in size, wasting almost half of the 52-bit VA space.
>
> Right, that is a huge wastage. Increasing the linear mapping will definitely
> help in getting more memory used on the system. But now do we have enough
> vmemmap range to support this enlarged linear mapping ?

Yes.