linux-kernel - Re: [PATCH] mm: clear 1G pages with streaming stores on x86

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Date:   Thu, 26 Mar 2020 12:46:11 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Salman Qazi <sqazi@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] mm: clear 1G pages with streaming stores on x86

On Mon, Mar 16, 2020 at 03:19:55PM +0300, Kirill A. Shutemov wrote:
> On Mon, Mar 16, 2020 at 11:18:56AM +0100, Michal Hocko wrote:
> > While this might be true, isn't that easily solveable by the existing
> > ALTERNATIVE and cpu features framework. Can we have a feature bit to
> > tell that movnti is worthwile for large data copy routines. Probably
> > something for x86 maintainers.
> 
> It still need somody to test which approach is better for the CPU.
> See X86_FEATURE_REP_GOOD.

How about inverting the sense of the bit?  Set it on Broadwell-and-
earlier CPUs, where it definitely improves performance by a huge amount,
and new microarches can set it if it still outperforms rep mov?