Messages in this thread | | | Date | Wed, 09 Jul 2008 14:47:51 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [RFC 00/15] x86_64: Optimize percpu accesses |
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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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