linux-kernel - Re: [RFC 00/17] Pile o' entry stack changes

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Message-ID: <20170907070446.uu2tzul45r76kj3o@gmail.com>
Date:   Thu, 7 Sep 2017 09:04:46 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Andi Kleen <andi@...stfloor.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC 00/17] Pile o' entry stack changes


* Andy Lutomirski <luto@...nel.org> wrote:

> On Wed, Sep 6, 2017 at 3:16 PM, Andi Kleen <andi@...stfloor.org> wrote:
> > Andy Lutomirski <luto@...nel.org> writes:
> >>
> >>  - Lots of people (Linus included) have asked to convert the entry
> >>    code to pop registers on exit instead of movqing them off the
> >>    stack.  This makes a bunch of progress in that direction.
> >
> > You should benchmark it on Atoms. Likely it's a regression there
> > because they don't have the special PUSH/POP acceleration.
> 
> I'm not entirely sure this is a worthwhile reason.  Atom will lose a
> few cycles due to POP throughput, but there's a lot less decode
> bandwidth needed and we save a cache line or two.

I think we can also safely assume that Atom will eventually either join the
21st century or die out - mild Atom micro-costs are not a good reason to
complicate the entry code...

Thanks,

	Ingo