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

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Date:   Thu, 21 Mar 2019 22:03:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     hpa@...or.com
Cc:     Denys Vlasenko <dvlasenk@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 11:18:05AM -0700, hpa@...or.com wrote:
> On March 21, 2019 10:25:05 AM PDT, Denys Vlasenko <dvlasenk@...hat.com> wrote:

> >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.
> 
> I think you will find that if you change system flags it is much slower.

So with all the patches in this series applied, only x86_32 will suffer
this, and I don't think anybody still considers that a performance
critical platform.

That said, we could do something terrible like:

--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -673,7 +673,8 @@ ENTRY(__switch_to_asm)
 #endif
 
        /* restore callee-saved registers */
-       popfl
+       ALTERNATIVE "popl %esi", \
+                   "popfl", X86_FEATURE_SMAP
        popl    %esi
        popl    %edi
        popl    %ebx

And then you only pay the POPF penalty when you run a 32bit kernel on a
SMAP enabled CPU, and we have a very good solution in that code: run a
64bit kernel.