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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 12 Feb 2020 07:23:16 +0100
Message-ID: <CACT4Y+aRq9j=3GODWBcnDnW=Pgp4e=N2++FTYEuq-00OmfXpbw@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, Feb 12, 2020 at 12:48 AM Patricia Alfonso
<trishalfonso@google.com> wrote:
>
> 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.

I think it's the right thing to do if it works.