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

[Jeremy Fitzhardinge]

Mike Travis wrote:
> 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");\
>   
       asm volatile("btr %2,%%gs:%1\n\tsbbl %0,%0"                    \
                    : "=r" (old__), "+m" (per_cpu_var(var))            \
                    : "dIr" (bit) : "memory");\


but it barely seems worthwhile if we really can't use test_and_clear_bit.

   J
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