On Wed, 6 Oct 2021 18:14:38 +0100
Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
>
> On Thu, Sep 30, 2021 at 06:50:26PM +0800, Jisheng Zhang wrote:
> > The 4KB pages + 3 levels (39-bit) combination is also widely used in
> > arm64 world, add the memory layout description for this combination.
> >
> > Signed-off-by: Jisheng Zhang <Jisheng.Zhang@xxxxxxxxxxxxx>
> > ---
> > Documentation/arm64/memory.rst | 17 +++++++++++++++++
> > 1 file changed, 17 insertions(+)
> >
> > diff --git a/Documentation/arm64/memory.rst b/Documentation/arm64/memory.rst
> > index 901cd094f4ec..d1745b570f0c 100644
> > --- a/Documentation/arm64/memory.rst
> > +++ b/Documentation/arm64/memory.rst
> > @@ -26,6 +26,23 @@ The swapper_pg_dir address is written to TTBR1 and never written to
> > TTBR0.
> >
> >
> > +AArch64 Linux memory layout with 4KB pages + 3 levels (39-bit)::
> > + Start End Size Use
> > + -----------------------------------------------------------------------
> > + 0000000000000000 0000007fffffffff 512GB user
> > + ffffff8000000000 ffffffbfffffffff 256GB kernel logical memory map
> > + [ffffffb000000000 ffffffbfffffffff] 64GB [kasan shadow region]
> > + ffffffc000000000 ffffffc007ffffff 128MB bpf jit region
> > + ffffffc008000000 ffffffc00fffffff 128MB modules
> > + ffffffc010000000 fffffffdefffffff 253440MB vmalloc
> > + fffffffdf0000000 fffffffdfdffffff 224MB fixed mappings (top down)
> > + fffffffdfe000000 fffffffdfe7fffff 8MB [guard region]
> > + fffffffdfe800000 fffffffdff7fffff 16MB PCI I/O space
> > + fffffffdff800000 fffffffdffffffff 8MB [guard region]
> > + fffffffe00000000 ffffffffefffffff 4GB vmemmap
> > + ffffffff00000000 ffffffffffffffff 4GB [guard region]
>
> I wouldn't bother maintaining these. There are other combinations that
> people may use. The 4KB + 48-bit VA is defconfig while 64KB + 52-bit was
> more interesting, so we thought it would be useful.
If kernel config file isn't based on the defconfig, the 4KB + 39bit VA is the
default option in Kconfig:
...
prompt "Page size"
default ARM64_4K_PAGES
help
Page size (translation granule) configuration.
...
choice
prompt "Virtual address space size"
default ARM64_VA_BITS_39 if ARM64_4K_PAGES
default ARM64_VA_BITS_47 if ARM64_16K_PAGES
default ARM64_VA_BITS_42 if ARM64_64K_PAGES
help
Allows choosing one of multiple possible virtual address
I believe this combination is widely used if 48bit VA isn't necessary. In
fact, I think the default android GKI image is built with 4KB + 39bit VA. This
is the reason why I added description for 4K + VA39 but leave other
combinations.
>
> I'm more inclined to remove them altogether and maybe just add some
> high-level ascii art as per the log of commit f4693c2716b3 ("arm64: mm:
> extend linear region for 52-bit VA configurations").
The detailed memory layout is useful, for example, I can immediately know
which region the virtual address belongs to when checking kernel panic logs.
If changed to high level ascii, I need to do some calculation, sometimes I
need to check source code to get the value of some MACROS defitions.
