linux-kernel - Re: [GIT PULL] x86 fixes

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Date:   Mon, 2 Jul 2018 11:47:53 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] x86 fixes

On Sat, Jun 30, 2018 at 12:01 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Sat, Jun 30, 2018 at 1:49 AM Ingo Molnar <mingo@...nel.org> wrote:
>>
>> --- a/arch/x86/entry/entry_32.S
>> +++ b/arch/x86/entry/entry_32.S
>> @@ -477,7 +477,7 @@ ENTRY(entry_SYSENTER_32)
>>          * whereas POPF does not.)
>>          */
>>         addl    $PT_EFLAGS-PT_DS, %esp  /* point esp at pt_regs->flags */
>> -       btr     $X86_EFLAGS_IF_BIT, (%esp)
>> +       btrl    $X86_EFLAGS_IF_BIT, (%esp)
>>         popfl
>
> Ho humm. Just looking at this patch, my reaction was "why isn't this
> an 'andl $~X86_EFLAGS_IF' instead"?
>
> Yeah, I guess the 'andl' is two bytes longer (due to the 32-bit
> constant - because IF is bit 9, you can't use a byte constant, and you
> don't want to get a partial word write just before the popfl).
>
> But btr is really pretty heavy operation for older CPU's (it's gotten
> better, but 32-bit code presumably cares more about the older CPUs).
>
> It really doesn't matter, I guess. The btr goes back to commit
> c2c9b52fab0d ("x86/entry/32: Restore FLAGS on SYSEXIT").
>
> Andy?
>

BTR is way more leet than AND!

Seriously, though, I've never really tried to shave cycles off the
32-bit code, and BTR is shorter, and I didn't spend more than about
one brain cycle thinking about it.  I guess that BTR has a more
complicated flags pipeline (the output flags depend on the input, not
just the output) and probably uses some more complicated ALU circuit
as compared to ANDL.

--Andy