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From:   Patricia Alfonso <trishalfonso@google.com>
Date:   Tue, 11 Feb 2020 15:48:18 -0800
Message-ID: <CAKFsvUKhwAOV9O+LWBr=-zLEJCFJvKOH-ePsXMMVJzHotqd3Ug@mail.gmail.com>
Subject: Re: [RFC PATCH] UML: add support for KASAN under x86_64
To:     Dmitry Vyukov <dvyukov@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 Thu, Jan 16, 2020 at 12:44 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> 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?
The only restriction is that there is enough space there to map all of
the KASAN shadow memory without conflicting with anything else.

> 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)
>
I just tested with 0x7fff8000 as the KASAN_SHADOW_OFFSET and it worked
so I can make that the default if it will be more efficient.

-- 
Patricia Alfonso