Messages in this thread | | | Date | Thu, 29 Nov 2007 11:08:41 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task |
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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.
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