linux-kernel - Re: [RFC 00/15] x86_64: Optimize percpu accesses

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Date:	Wed, 09 Jul 2008 14:39:41 -0700
From:	Mike Travis <travis@....com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Eric W. Biederman wrote:
> I just took a quick look at how stack_protector works on x86_64.  Unless there is
> some deep kernel magic that changes the segment register to %gs from the ABI specified
> %fs CC_STACKPROTECTOR is totally broken on x86_64.  We access our pda through %gs.
> 
> Further -fstack-protector-all only seems to detect against buffer overflows and
> thus corruption of the stack.  Not stack overflows.  So it doesn't appear especially
> useful.
> 
> So we don't we kill the broken CONFIG_CC_STACKPROTECTOR.  Stop trying to figure out
> how to use a zero based percpu area.
> 
> That should allow us to make the current pda a per cpu variable, and use %gs with
> a large offset to access the per cpu area.  And since it is only the per cpu accesses
> and the pda accesses that will change we should not need to fight toolchain issues
> and other weirdness.  The linked binary can remain the same.
> 
> Eric

Hi Eric,

There is one pda op that I was not able to remove.  Most likely it can be recoded
but it was a bit over my expertise.  Most likely the "pda_offset(field)" can be
replaced with "per_cpu_var(field)" [per_cpu__##field], but for "_proxy_pda.field"
I wasn't sure about.

include/asm-x86/pda.h:

/*
 * This is not atomic against other CPUs -- CPU preemption needs to be off
 * NOTE: This relies on the fact that the cpu_pda is the *first* field in
 *       the per cpu area.  Move it and you'll need to change this.
 */
#define test_and_clear_bit_pda(bit, field)                              \
({                                                                      \
        int old__;                                                      \
        asm volatile("btr %2,%%gs:%c3\n\tsbbl %0,%0"                    \
                     : "=r" (old__), "+m" (_proxy_pda.field)            \
                     : "dIr" (bit), "i" (pda_offset(field)) : "memory");\
        old__;                                                          \
})

And there is only one reference to it.  

arch/x86/kernel/process_64.c:

static void __exit_idle(void)
{
        if (test_and_clear_bit_pda(0, isidle) == 0)
                return;
        atomic_notifier_call_chain(&idle_notifier, IDLE_END, NULL);
}

Thanks,
Mike
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