Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <55176b79-90af-4a47-dc06-9f5f2f2c123d@intel.com>
Date:   Thu, 12 May 2022 14:51:31 -0700
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.8.1
Subject: Re: [RFCv2 00/10] Linear Address Masking enabling
Content-Language: en-US
To:     Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>, x86@kernel.org,
        Alexander Potapenko <glider@google.com>,
        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-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20220511022751.65540-1-kirill.shutemov@linux.intel.com>
 <20220511064943.GR76023@worktop.programming.kicks-ass.net>
 <20bada85-9203-57f4-2502-57a6fd11f3ea@intel.com> <875ymav8ul.ffs@tglx>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <875ymav8ul.ffs@tglx>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

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?