Re: [RFC PATCH] UML: add support for KASAN under x86_64

[Dmitry Vyukov]

On Wed, Jan 15, 2020 at 7:28 PM Patricia Alfonso
<trishalfonso@xxxxxxxxxx> wrote:
> +config KASAN_SHADOW_OFFSET
> +       hex
> +       depends on KASAN
> +       default 0x100000000000
> +       help
> +         This is the offset at which the ~2.25TB of shadow memory is
> +         initialized and used by KASAN for memory debugging. The default
> +         is 0x100000000000.

What are restrictions on this value?
In user-space we use 0x7fff8000 as a base (just below 2GB) and it's
extremely profitable wrt codegen since it fits into immediate of most
instructions.
We can load and add the base with a short instruction:
    2d8c: 48 81 c2 00 80 ff 7f    add    $0x7fff8000,%rdx
Or even add base, load shadow and check it with a single 7-byte instruction:
     1e4: 80 b8 00 80 ff 7f 00    cmpb   $0x0,0x7fff8000(%rax)

While with the large base, it takes 10 bytes just to load the const
into a register (current x86 KASAN codegen):
ffffffff81001571: 48 b8 00 00 00 00 00 fc ff df    movabs
$0xdffffc0000000000,%rax
Most instructions don't have 8-byte immediates, so then we separately
need to add/load/check.