* Tejun Heo <tj@xxxxxxxxxx> wrote: > 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. Yeah - if then this should definitely be handled in the compiler. Ingo -- 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
|