linux-kernel - Re: [PATCHv4 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED

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Date:   Thu, 14 Jul 2022 16:28:36 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        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: [PATCHv4 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR

On Tue, Jul 12, 2022 at 7:14 PM Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> On Tue, Jul 12, 2022 at 03:12:01PM +0200, Alexander Potapenko wrote:
> > On Wed, Jun 22, 2022 at 6:22 PM Kirill A. Shutemov
> > <kirill.shutemov@...ux.intel.com> wrote:
> > >
> > > Add a couple of arch_prctl() handles:
> > >
> > >  - ARCH_ENABLE_TAGGED_ADDR enabled LAM. The argument is required number
> > >    of tag bits. It is rounded up to the nearest LAM mode that can
> > >    provide it. For now only LAM_U57 is supported, with 6 tag bits.
> > >
> > >  - ARCH_GET_UNTAG_MASK returns untag mask. It can indicates where tag
> > >    bits located in the address.
> > >
> > Am I right that the desired way to detect the presence of LAM without
> > enabling it is to check that arch_prctl(ARCH_GET_UNTAG_MASK, ...)
> > returns zero?
>
> Returns -1UL, but yes.

No, I meant the return value of arch_prctl(), but in fact neither
seems to be true.

Right now e.g. for the 5.17 kernel arch_prctl(ARCH_GET_UNTAG_MASK,
&bits) returns -EINVAL regardless of the underlying hardware.
A new kernel with your patches will return 0 and set bits=-1UL on both
non-LAM and LAM-enabled machines. How can we distinguish those?

> >
> > One would expect that `arch_prctl(ARCH_ENABLE_TAGGED_ADDR, 0)`
> > disables tagging for the current process.
> > Shouldn't this workflow be supported as well?
>
> Is there an use-case for it?
>
> I would rather keep the interface minimal. We can always add this in the
> future if an use-case comes.

As discussed offline, we don't have a use-case for this yet, so I don't insist.

> --
>  Kirill A. Shutemov



-- 
Alexander Potapenko
Software Engineer

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