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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 10 Jul 2020 15:37:10 -0700
Message-ID: <CAHk-=wjQJjZX3cgX6vLp6ntTiBG+a6md64J+e74SDntoTi+q4A@mail.gmail.com>
Subject: Re: objtool clac/stac handling change..
To:     David Laight <David.Laight@aculab.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Jul 7, 2020 at 5:35 AM David Laight <David.Laight@aculab.com> wrote:
>
>
> So separate copy and checksum passes should easily exceed 4 bytes/clock,
> but I suspect that doing them together never does.
> (Unless the buffer is too big for the L1 cache.)

Its' the "touch the caches twice" that is the problem".

And it's not the "buffer is too big for L1", it's "the source, the
destination and any incidentals are too big for L1" with the
additional noise from replacement policies etc.

That said, I agree it's likely less of an issue these days when L1
sizes are bigger, and thankfully direct-mapped caches are no more. It
_used_ to be that touching the location twice was very very noticeable
in some situations, it may not be so much any more.

             Linus