linux-kernel - Re: [PATCHv1, RFC 0/8] Boot-time switching between 4- and 5-level paging

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Date:   Fri, 26 May 2017 18:58:12 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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@...r.kernel.org" <linux-arch@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv1, RFC 0/8] Boot-time switching between 4- and 5-level
 paging

On Fri, May 26, 2017 at 08:51:48AM -0700, Linus Torvalds wrote:
> On Fri, May 26, 2017 at 6:00 AM, Kirill A. Shutemov
> <kirill@...temov.name> wrote:
> >
> > I don't see how kernel threads can use 4-level paging. It doesn't work
> > from virtual memory layout POV. Kernel claims half of full virtual address
> > space for itself -- 256 PGD entries, not one as we would effectively have
> > in case of switching to 4-level paging. For instance, addresses, where
> > vmalloc and vmemmap are mapped, are not canonical with 4-level paging.
> 
> I would have just assumed we'd map the kernel in the shared part that
> fits in the top 47 bits.
> 
> But it sounds like you can't switch back and forth anyway, so I guess it's moot.
> 
> Where *is* the LA57 documentation, btw? I had an old x86 architecture
> manual, so I updated it, but LA57 isn't mentioned in the new one
> either.

It's in a separate white paper for now:

https://software.intel.com/sites/default/files/managed/2b/80/5-level_paging_white_paper.pdf

-- 
 Kirill A. Shutemov