linux-kernel - Re: [PATCH 01/25] x86: Make SMAP 64-bit only

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Date:   Thu, 21 Mar 2019 11:21:05 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Denys Vlasenko <dvlasenk@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Anvin <hpa@...or.com>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 01/25] x86: Make SMAP 64-bit only

On Thu, Mar 21, 2019 at 10:25 AM Denys Vlasenko <dvlasenk@...hat.com> wrote:
>
> >
> > But iirc pushf/popf isn't really that expensive - in fact I think it's
> > pretty cheap when system flags don't change.
>
> I did not see evidence of this. In my testing,
> POPF is always ~20 cycles, even if popped flags are identical to current
> state of flags.

It may have been an artifact on just some older CPU's. I have this
distinct memory of popf that changed IF being more expensive, but
maybe that was the Pentium 4 days.

Or maybe it's just that my memory is garbage.

               Linus