Re: [PATCH 1/3] x86: drop unnecessary kernel_eflags variable from 64 bit.

From: Pekka Enberg <penberg@kernel.org>
To: Ian Campbell <ian.campbell@citrix.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>, Brian Gerst <brgerst@gmail.com>,
	Jan Beulich <JBeulich@novell.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	x86@kernel.org, Andi Kleen <ak@linux.intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 1/3] x86: drop unnecessary kernel_eflags variable from 64 bit.
Date: Wed, 6 Jul 2011 12:39:56 +0300	[thread overview]
Message-ID: <CAOJsxLFjdrrnx4gZjDJQuf3GBHeKvKb91TajYBkMu+PgSWtM5A@mail.gmail.com> (raw)
In-Reply-To: <1309870814-14181-1-git-send-email-ian.campbell@citrix.com>

On Tue, Jul 5, 2011 at 4:00 PM, Ian Campbell <ian.campbell@citrix.com> wrote:
> For no reason that I can determine 64 bit x86 saves the current eflags
> in cpu_init purely for use in ret_from_fork. The equivalent 32 bit
> code simply hard codes 0x0202 as the new EFLAGS which seems safer than
> relying on a potentially arbitrary EFLAGS saved during cpu_init.
>
> Original i386 changeset
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=47a5c6fa0e204a2b63309c648bb2fde36836c826
> Original x86_64 changset
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=658fdbef66e5e9be79b457edc2cbbb3add840aa9
>
> The only comment in the later indicates that it is following the
> former, but not why it differs in this way.

Well lets ask Linus and Andi?

> This change makes 64 bit use the same mechanism to setup the initial
> EFLAGS on fork. Note that 64 bit resets EFLAGS before calling
> schedule_tail() as opposed to 32 bit which calls schedule_tail()
> first. Therefore the correct value for EFLAGS has opposite IF
> bit. This will be fixed in a subsequent patch.
>
> Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
> Cc: x86@kernel.org
> ---
>  arch/x86/kernel/cpu/common.c |    4 ----
>  arch/x86/kernel/entry_64.S   |    2 +-
>  2 files changed, 1 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 22a073d..178c3b4 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1077,8 +1077,6 @@ void syscall_init(void)
>               X86_EFLAGS_TF|X86_EFLAGS_DF|X86_EFLAGS_IF|X86_EFLAGS_IOPL);
>  }
>
> -unsigned long kernel_eflags;
> -
>  /*
>  * Copies of the original ist values from the tss are only accessed during
>  * debugging, no special alignment required.
> @@ -1231,8 +1229,6 @@ void __cpuinit cpu_init(void)
>        fpu_init();
>        xsave_init();
>
> -       raw_local_save_flags(kernel_eflags);
> -
>        if (is_uv_system())
>                uv_cpu_init();
>  }
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index e13329d..b50d142 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -396,7 +396,7 @@ ENTRY(ret_from_fork)
>
>        LOCK ; btr $TIF_FORK,TI_flags(%r8)
>
> -       pushq_cfi kernel_eflags(%rip)
> +       pushq_cfi $0x0002
>        popfq_cfi                               # reset kernel eflags
>
>        call schedule_tail                      # rdi: 'prev' task parameter