Ok the sign bit doesn't really make any sense on second thought... to work with set_fs() we have to load something from memory anyway and then we might as well do a compare... "H. Peter Anvin" <hpa@xxxxxxxxx> wrote: >On 12/26/2013 11:00 AM, Linus Torvalds wrote: >> >> Interestingly, looking at the cp_new_stat() profiles, the games we >> play to get efficient range checking seem to actually hurt us. Maybe >> it's the "sbb" that is just expensive, or maybe it's turning a (very >> predictable) conditional branch into a data dependency chain instead. >> Or maybe it's just random noise in my profiles that happened to make >> those sbb's look bad. >> > >I'm not at all surprised... there is a pretty serious data dependency >chain here and in the end we end up manifesting a value in a register >that has to be tested even though it is available in the flags. Inline >assembly also means the compiler can't optimize it at all. > >I have to wonder if we actually have to test the upper limit, though: >we >can always guarantee a guard zone between user space and kernel space, >and thus guarantee either a #PF or #GP if someone tries to overflow >user >space. Testing just the lower limit would be much cheaper, especially >on 64 bits where we can simply test the sign bit. > >What do you think? > > -hpa -- Sent from my mobile phone. Please pardon brevity and lack of formatting. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
