Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task

H. Peter Anvin
2007-11-29T19:17:59Z

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