linux-kernel - Re: [PATCHv5 00/13] Linear Address Masking enabling

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Date:   Wed, 20 Jul 2022 03:59:50 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Alexander Potapenko <glider@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Taras Madan <tarasmadan@...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 Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv5 00/13] Linear Address Masking enabling

On Mon, Jul 18, 2022 at 07:39:22PM +0200, Alexander Potapenko wrote:
> On Wed, Jul 13, 2022 at 1:13 AM Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
> >
> > Linear Address Masking[1] (LAM) modifies the checking that is applied to
> > 64-bit linear addresses, allowing software to use of the untranslated
> > address bits for metadata.
> >
> > The patchset brings support for LAM for userspace addresses.
> >
> > LAM_U48 enabling is controversial since it competes for bits with
> > 5-level paging. Its enabling isolated into an optional last patch that
> > can be applied at maintainer's discretion.
> 
> I believe having optional patches will put unnecessary burden on
> distro maintainers.
> Soon after landing U48 support other changes will start piling on top
> of it, and it will be impossible to maintain a kernel with this patch
> removed.
> It also won't make any difference for the upstream, where this patch
> will be always present.
> 
> We'd better decide now whether we need U48 or not, and either keep it
> or delete it.

Dave, Andy, any position on this?

I wrote LAM_U48 support to prove that interface is flexible enough, but I
see why it can be a problem if a distro will pick them up ahead of
upstream.

-- 
 Kirill A. Shutemov