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Date:   Fri, 27 May 2022 13:28:12 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Mark Hemment <markhemm@googlemail.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Patrice CHOTARD <patrice.chotard@foss.st.com>,
        Mikulas Patocka <mpatocka@redhat.com>,
        Lukas Czerner <lczerner@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Chuck Lever <chuck.lever@oracle.com>,
        Hugh Dickins <hughd@google.com>, patches@lists.linux.dev,
        Linux-MM <linux-mm@kvack.org>, mm-commits@vger.kernel.org,
        Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH] x86/clear_user: Make it faster
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On Wed, May 25, 2022 at 01:11:17PM +0100, Mark Hemment wrote:
> A slight doubt here; comment says "less than a cachline", but the code
> is using 'ja' (jump if above) - so calls 'clear_user_original' for a
> 'len' less than or equal to 64.
> Not sure of the intended behaviour for 64 bytes here, but
> 'copy_user_enhanced_fast_string' uses the slow-method for lengths less
> than 64.  So, should this be coded as;
>     cmp $64,%rcx
>     jb clear_user_original
> ?

Yeah, it probably doesn't matter whether you clear a cacheline the "old"
way or with some of the new ones. clear_user() performance matters only
in microbenchmarks, as I've come to realize.

But your suggestion simplifies the code so lemme do that.

Thx!

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette