linux-kernel - [Q x86-64] on kernel

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Message-ID: <20110705104742.GO17941@sun>
Date:	Tue, 5 Jul 2011 14:47:42 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Brian Gerst <brgerst@...il.com>,
	Jan Beulich <JBeulich@...ell.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: [Q x86-64] on kernel_eflags

While were looking into ret_from_fork code I somehow wondered
about the global kernel_eflags variable here.

arch/x86/cpu/common.c
---------------------
unsigned long kernel_eflags;

void __cpuinit cpu_init(void)
{
	...
	raw_local_save_flags(kernel_eflags);
}

arch/x86/kernel/entry_64.S
--------------------------
ENTRY(ret_from_fork)
	DEFAULT_FRAME
	LOCK ; btr $TIF_FORK,TI_flags(%r8)
	pushq_cfi kernel_eflags(%rip)
	popfq_cfi                               # reset kernel eflags

Every call to cpu_init renew global kernel_eflags
and every task switching does use this variable in a
sake of cleaning carry bit of flags register as far as
I can tell.

Should not every cpu has own copy of kernel_eflags? Just
to be consistent in style? Or this would be space waisting
and an optimization is done here?

	Cyrill
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