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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 16 Jan 2020 09:44:08 +0100
Message-ID: <CACT4Y+bPzRbWw-dPQkLVENPKy_DBdjrbSce0f6XE3=W7RhfhBA@mail.gmail.com>
Subject: Re: [RFC PATCH] UML: add support for KASAN under x86_64
To:     Patricia Alfonso <trishalfonso@google.com>
Cc:     Jeff Dike <jdike@addtoit.com>, Richard Weinberger <richard@nod.at>,
        anton.ivanov@cambridgegreys.com,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        David Gow <davidgow@google.com>,
        Brendan Higgins <brendanhiggins@google.com>,
        linux-um@lists.infradead.org,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Jan 15, 2020 at 7:28 PM Patricia Alfonso
<trishalfonso@google.com> 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.