linux-kernel - Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync

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Date:   Fri, 2 Dec 2016 09:53:52 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Peter Anvin <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Borislav Petkov <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Matthew Whitehead <tedheadster@...il.com>,
        Henrique de Moraes Holschuh <hmh@....eng.br>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation

On Fri, Dec 2, 2016 at 9:38 AM, Andy Lutomirski <luto@...nel.org> wrote:
>
> apply_alternatives, unfortunately.  It's performance-critical because
> it's intensely stupid and does sync_core() for every single patch.
> Fixing that would be nice, too.

So looking at text_poke_early(), that's very much a case that really
shouldn't need any "sync_core()" at all as far as I can tell.

Only the current CPU is running, and for local CPU I$ coherence all
you need is a jump instruction, and even that is only on really old
CPU's. From the PPro onwards (maybe even Pentium?) the I$ is entirely
serialized as long as you change the data using the same linear
address.

So at most, that function could mark itsel f"noinline" just to
guarantee that it will cause a control flow change before returning.
The sync_core() seems entirely bogus.

Same goes for optimize_nops() too.

              Linus