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Date:   Sat, 14 May 2022 02:01:23 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Alexander Potapenko <glider@google.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        "H . J . Lu" <hjl.tools@gmail.com>,
        Andi Kleen <ak@linux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFCv2 00/10] Linear Address Masking enabling
Message-ID: <20220513230123.wmb4ia3r72snfpwj@black.fi.intel.com>
References: <20220511022751.65540-1-kirill.shutemov@linux.intel.com>
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On Fri, May 13, 2022 at 01:07:43PM +0200, Alexander Potapenko wrote:
> On Thu, May 12, 2022 at 11:51 PM Dave Hansen <dave.hansen@intel.com> wrote:
> >
> > On 5/12/22 12:39, Thomas Gleixner wrote:
> > >> It's OK for a debugging build that runs on one kind of hardware.  But,
> > >> if we want LAM-using binaries to be portable, we have to do something
> > >> different.
> > >>
> > >> One of the stated reasons for adding LAM hardware is that folks want to
> > >> use sanitizers outside of debugging environments.  To me, that means
> > >> that LAM is something that the same binary might run with or without.
> > > On/off yes, but is there an actual use case where such a mechanism would
> > > at start time dynamically chose the number of bits?
> >
> > I'd love to hear from folks doing the userspace side of this.  Will
> > userspace be saying: "Give me all the bits you can!".  Or, will it
> > really just be looking for 6 bits only, and it doesn't care whether it
> > gets 6 or 15, it will use only 6?
> 
> (speaking more or less on behalf of the userspace folks here)
> I think it is safe to assume that in the upcoming year or two HWASan
> will be fine having just 6 bits for the tags on x86 machines.
> We are interested in running it on kernels with and without
> CONFIG_X86_5LEVEL=y, so U48 is not an option in some cases anyway.

Just to be clear: LAM_U48 works on machine with 5-level paging enabled as
long as the process doesn't map anything above 47-bit.

-- 
 Kirill A. Shutemov