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

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Message-ID: <878rr6v985.ffs@tglx>
Date:   Thu, 12 May 2022 21:31:22 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Alexander Potapenko <glider@...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,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [RFCv2 00/10] Linear Address Masking enabling

On Thu, May 12 2022 at 19:56, Kirill A. Shutemov wrote:
> On Thu, May 12, 2022 at 05:42:58PM +0200, Thomas Gleixner wrote:
>> On Wed, May 11 2022 at 08:49, Peter Zijlstra wrote:
>> > On Wed, May 11, 2022 at 05:27:40AM +0300, Kirill A. Shutemov wrote:
>> > So aren't we creating a problem with LAM_U48 where programs relying on
>> > it are of limited sustainability?
>> >
>> > Any such program simply *cannot* run on 5 level pagetables. Why do we
>> > want to do this?
>> 
>> More bits are better :)
>> 
>> Seriously, I agree that restricting it to LAM57, which gives us 6 bits,
>> makes a lot of sense _and_ makes the whole thing way simpler.
>> 
>> So supporting both needs a truly good justification and a real world use
>> case.
>
> I asked the question before[1]. Basically, more bits more better:
>
> 	For HWASAN #bits == detection probability.
> 	For MarkUS #bits == exponential cost reduction

What is MarkUS? It's not really helpful to provide acronyms which are
not decodable.

> I would really like to have only LAM_U57, but IIUC 6 bits is not always
> enough.
>
> Dmitry, could you elaborate?
>
> [1] https://mobile.twitter.com/dvyukov/status/1342019823400837120

I don't know whether he reacts on posting a link to his twitter
account. I've CC'ed him now. Maybe that works better.

Thanks,

        tglx