linux-kernel - Re: [PATCHv1, RFC 5/8] x86/mm: Fold p4d page table layer at runtime

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Date:   Sun, 28 May 2017 01:46:24 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Brian Gerst <brgerst@...il.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv1, RFC 5/8] x86/mm: Fold p4d page table layer at runtime

On Sat, May 27, 2017 at 11:09:54AM -0400, Brian Gerst wrote:
> >  static inline int pgd_none(pgd_t pgd)
> >  {
> > +       if (p4d_folded)
> > +               return 0;
> >         /*
> >          * There is no need to do a workaround for the KNL stray
> >          * A/D bit erratum here.  PGDs only point to page tables
> 
> These should use static_cpu_has(X86_FEATURE_LA57), so that it gets
> patched by alternatives.

Right, eventually we would likely need something like this. But at this
point I'm more worried about correctness than performance. Performance
will be the next step.

And I haven't tried it yet, but I would expect direct use of alternatives
wouldn't be possible. If I read code correctly, we enable paging way
before we apply alternatives. But we need to have something functional in
between.

I guess it will be fun :)

-- 
 Kirill A. Shutemov