Hello, H. Peter Anvin wrote: > On 09/03/2009 01:45 PM, Jeremy Fitzhardinge wrote: >> Two problems: >> >> * gcc generates %gs: references for stack-protector, but we use %fs >> for percpu data (because restoring %fs is faster if it's a null >> selector; TLS uses %gs). I guess we could use %fs if >> !CONFIG_CC_STACKPROTECTOR, or %gs if we are using it (though that >> has some fiddly ramifications for things like ptrace). > > Well, by touching two segments we're getting the worst of both worlds, > so at least assuming some significant number of real-world deployments > use CC_STACKPROTECTOR, we really don't want to pessimize that case too much. Yes, this one definitely seems doable. BTW, how much performance does CC_STACKPROTECTOR cost? That's an ambiguous question but really any number would help to develop a general sense. Considering fedora is doing it by default, I assume it isn't too high? >> * The i386 percpu %fs base is offset by -__per_cpu_start from the >> percpu variables, so we can directly refer to %fs:per_cpu__foo. >> I'm not sure what it would take to unify i386 to use the same >> scheme as x86-64. > > OK, I was under the impression that that had already been done (and no, > I didn't bother to look at the code.) I guess I was wrong (and yes, > this is an absolute precondition.) I tried this a while ago but hit an obstacle which I don't remember what exactly was now and decided the conversion wasn't worth the trouble. IIRC, it was something substantial. I'll dig through my trees. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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