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

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Date:   Mon, 16 Sep 2019 16:53:05 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "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>,
        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


* Dave Hansen <dave.hansen@...el.com> wrote:

> 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>

Ok - in terms of timing it's obviously *way* too late for v5.4, so I've 
queued it up for the v5.5 merge window in tip:x86/mm. This should give it 
2-3 months of additional testing to shake out any weird interactions and 
quirks.

Thanks,

	Ingo