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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 30 Jun 2018 12:01:20 -0700
Message-ID: <CA+55aFz4BiRVKKwdF+y8K4O1jZnig8mmuQZ7prGp6o+DiH_TeQ@mail.gmail.com>
Subject: Re: [GIT PULL] x86 fixes
To:     Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@amacapital.net>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Andrew Morton <akpm@linux-foundation.org>
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On Sat, Jun 30, 2018 at 1:49 AM Ingo Molnar <mingo@kernel.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?

                Linus