linux-kernel - Re: [RFCv2 00/10] Linear Address Masking enabling

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Date:   Thu, 12 May 2022 14:51:31 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFCv2 00/10] Linear Address Masking enabling

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?