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 11:24:39 -0700
From:   hpa@...or.com
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>
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>,
        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 May 26, 2017 8:51:48 AM PDT, Linus Torvalds <torvalds@...ux-foundation.org> 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.
>
>                       Linus

As one of the major motivations for LA57 is that we expect that we will have machines with more than 2^46 bytes of memory in the near future, it isn't feasible in most cases to do per-VM LA57.

The only case where that even has any utility is for an application to want more than 128 TiB address space on a machine with no more than 64 TiB of RAM.  It is kind of a narrow use case, I think.
-- 
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