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SubjectRe: [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.


* 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.


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


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