H. Peter Anvin wrote: > On 09/03/2009 08:47 PM, Tejun Heo wrote: >> H. Peter Anvin wrote: >>> On 09/03/2009 07:59 PM, Tejun Heo wrote: >>>> Another question. Other than saving and loading an extra segment >>>> register on kernel entry/exit, whether using the same or different >>>> segment registers doesn't look like would make difference >>>> performance-wise. If I'm interpreting the wording in the optimization >>>> manual correctly, it means that each non-zero segment based memory >>>> access will be costly regardless of which specific segment register is >>>> in use and there's no way we can merge segment based dereferences for >>>> stackprotector and percpu variables. >>>> >>> It's correct that it doesn't make any difference for access, only for load. >> Heh... here's a naive and hopeful plan. How about we beg gcc >> developers to allow different segment register and offset in newer gcc >> versions and then use the same one when building with the new gcc? >> This should solve the i386 problem too. It would be the best as we >> get to keep the separate segment register from the userland. Too >> hopeful? > > I think it's possible to set the register in more recent gcc. Doing the > sane thing and having a symbol for an offset is probably worse. I was thinking about altering the build process so that we can use sed to substitute %gs:40 with %fs:40 while compiling. If it's already possible to override the register in more recent gcc, no need to go into that horror. > I can talk to H.J. Lu about this tomorrow. Great, please keep us posted. 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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