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Andi, do you happen to remember the details on this? -hpa Linus Torvalds wrote: > > On Thu, 29 Nov 2007, H. Peter Anvin wrote: >> Linus Torvalds wrote: >>>> It is advantageous for user space to use the register the kernel typically >>>> won't, in order to speed up system call entry/exit. >>> but I'm not seeing the reason for that one. Care to comment more? (Yes, >>> there is often a latency from segment reload to use, but the reload latency >>> for system call exit *should* be entirely covered by the cost of doing the >>> system call return itself, no?) >> I do seem to recall that some processor implementations can load a NULL >> segment faster than a non-NULL segment. This was significant enough that we >> wanted to use %fs in x86-64 userspace, as opposed to the original ABI which >> used %gs both in userspace and in the kernel. > > Ahh, I think you may be right for some CPUs. The zero selector is indeed > potentially faster to load, since it doesn't have to even bother looking > at the GDT/LDT. > > That said, I doubt it's very noticeable. I just ran tests on both an old > P4 and on a more modern Core 2 machine, and for both of those the > performance was identical between loading a NUL selector and loading it > with a non-zero one. > > But I could well imagine that it matters a few cycles on other CPU's. But > from my testing, it definitely isn't noticeable, and I think the > maintenance advantage of using the same segment setup would more than make > up for the fact that maybe some odd CPU can see a difference. > > Linus - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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