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Date:   Mon, 21 Sep 2020 18:43:59 +0100
From:   Will Deacon <will@kernel.org>
To:     Alexander Potapenko <glider@google.com>
Cc:     Marco Elver <elver@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christoph Lameter <cl@linux.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dmitriy Vyukov <dvyukov@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Hillf Danton <hdanton@sina.com>,
        Ingo Molnar <mingo@redhat.com>, Jann Horn <jannh@google.com>,
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Subject: Re: [PATCH v3 03/10] arm64, kfence: enable KFENCE for ARM64
Message-ID: <20200921174357.GB3141@willie-the-truck>
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On Mon, Sep 21, 2020 at 05:37:10PM +0200, Alexander Potapenko wrote:
> On Mon, Sep 21, 2020 at 4:58 PM Alexander Potapenko <glider@google.com> wrote:
> >
> > On Mon, Sep 21, 2020 at 4:31 PM Will Deacon <will@kernel.org> wrote:
> > >
> > > On Mon, Sep 21, 2020 at 03:26:04PM +0200, Marco Elver wrote:
> > > > Add architecture specific implementation details for KFENCE and enable
> > > > KFENCE for the arm64 architecture. In particular, this implements the
> > > > required interface in <asm/kfence.h>. Currently, the arm64 version does
> > > > not yet use a statically allocated memory pool, at the cost of a pointer
> > > > load for each is_kfence_address().
> > > >
> > > > Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> > > > Co-developed-by: Alexander Potapenko <glider@google.com>
> > > > Signed-off-by: Alexander Potapenko <glider@google.com>
> > > > Signed-off-by: Marco Elver <elver@google.com>
> > > > ---
> > > > For ARM64, we would like to solicit feedback on what the best option is
> > > > to obtain a constant address for __kfence_pool. One option is to declare
> > > > a memory range in the memory layout to be dedicated to KFENCE (like is
> > > > done for KASAN), however, it is unclear if this is the best available
> > > > option. We would like to avoid touching the memory layout.
> > >
> > > Sorry for the delay on this.
> >
> > NP, thanks for looking!
> >
> > > Given that the pool is relatively small (i.e. when compared with our virtual
> > > address space), dedicating an area of virtual space sounds like it makes
> > > the most sense here. How early do you need it to be available?
> >
> > Yes, having a dedicated address sounds good.
> > We're inserting kfence_init() into start_kernel() after timekeeping_init().
> > So way after mm_init(), if that matters.
> 
> The question is though, how big should that dedicated area be?
> Right now KFENCE_NUM_OBJECTS can be up to 16383 (which makes the pool
> size 64MB), but this number actually comes from the limitation on
> static objects, so we might want to increase that number on arm64.

What happens on x86 and why would we do something different?

Will