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From:   "H.J. Lu" <hjl.tools@gmail.com>
Date:   Thu, 12 May 2022 15:10:17 -0700
Message-ID: <CAMe9rOqb6ZnAZYe4uAWDt-vmhhP=z_+uZwi5fBURqyUWxCX9Cg@mail.gmail.com>
Subject: Re: [RFCv2 00/10] Linear Address Masking enabling
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        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 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.   An application
can ask for LAM_U48 or LAM_U57.  But the decision should be made by
application.  When an application asks for LAM_U57, I expect it will store
tags in upper 6 bits, even if the kernel enables LAM_U48.

-- 
H.J.