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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.

    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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