linux-kernel - Re: [PATCH] x86/mm: Enable 5-level paging support by default

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Message-ID: <8435951c-d88a-a5c6-5328-90c9f2521664@intel.com>
Date:   Fri, 13 Sep 2019 11:20:26 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mm: Enable 5-level paging support by default

On 9/13/19 2:54 AM, Kirill A. Shutemov wrote:
> The next major release of distributions expected to have
> CONFIG_X86_5LEVEL=y.

It's probably worth noting that this exposes to two kinds of possible
performance issues:

First is the overhead of having the 5-level code on 4-level hardware.
We haven't seen any regressions there in quite a while.  Kirill talked
about this in the changelog.

Second is the overhead of having 5-level paging active on 5-level
hardware versus using 4-level paging on hardware *capable* of 5-level.
That is, of course, much harder to measure since 5-level hardware is not
publicly available.  But, we've tested this quite a bit and we're pretty
confident that it will not cause regressions, especially on systems
where apps don't opt in to the larger address space.

I do think endeavoring to have mainline's defaults match the most common
distro configs is a good idea, and now is as good of a time as any.

Acked-by: Dave Hansen <dave.hansen@...ux.intel.com>