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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Dave Hansen <dave.hansen@intel.com>,
        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
Subject: Re: [RFCv2 00/10] Linear Address Masking enabling
In-Reply-To: <20bada85-9203-57f4-2502-57a6fd11f3ea@intel.com>
References: <20220511022751.65540-1-kirill.shutemov@linux.intel.com>
 <20220511064943.GR76023@worktop.programming.kicks-ass.net>
 <20bada85-9203-57f4-2502-57a6fd11f3ea@intel.com>
Date:   Thu, 12 May 2022 21:39:30 +0200
Message-ID: <875ymav8ul.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Thu, May 12 2022 at 10:22, Dave Hansen wrote:

> On 5/10/22 23:49, Peter Zijlstra wrote:
>>> The feature competes for bits with 5-level paging: LAM_U48 makes it
>>> impossible to map anything about 47-bits. The patchset made these
>>> capability mutually exclusive: whatever used first wins. LAM_U57 can be
>>> combined with mappings above 47-bits.
>> So aren't we creating a problem with LAM_U48 where programs relying on
>> it are of limited sustainability?
>
> I think allowing apps to say, "It's LAM_U48 or bust!" is a mistake.

That'd be outright stupid.

> 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?

> It's totally fine with me if the kernel only initially supports LAM_U57.
>  But, I'd ideally like to make sure that the ABI can support LAM_U57,
> LAM_U48, AMD's UAI (in whatever form it settles), or other masks.

Sure. No argument here.

Thanks,

        tglx