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From:   "H.J. Lu" <hjl.tools@gmail.com>
Date:   Thu, 12 May 2022 17:08:35 -0700
Message-ID: <CAMe9rOpXOLEMcir9zMq_UJe08Y-kM+9zok6gDicqAhPySV+3NA@mail.gmail.com>
Subject: Re: [RFCv2 00/10] Linear Address Masking enabling
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andi Kleen <ak@linux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, May 12, 2022 at 4:35 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Thu, May 12 2022 at 15:10, H. J. Lu wrote:
> > On Thu, May 12, 2022 at 2:51 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?
> >>
> >> Do the sanitizers have more overhead with more bits?  Or *less* overhead
> >> because they can store more metadata in the pointers?
> >>
> >> Will anyone care about the difference about potentially missing 1/64
> >> issues with U57 versus 1/32768 with U48?
> >
> > The only LAM usage I know so far is LAM_U57 in HWASAN.
>
> That's at least a halfways useful answer.
>
> > An application can ask for LAM_U48 or LAM_U57. But the decision should
> > be made by application.
>
> It can ask for whatever, but the decision whether it's granted is made
> by the kernel for obvious reasons.
>
> > When an application asks for LAM_U57, I expect it will store tags in
> > upper 6 bits, even if the kernel enables LAM_U48.
>
> The kernel does not enable LAM_U48 when the application only wants to
> have LAM_U57, because that would restrict the address space of the
> application to 47 bits on 5-level capable system for no reason.
>
> So what are you trying to tell me?
>

I am expecting applications to ask for LAM_U48 or LAM_U57, not just
LAM.

-- 
H.J.