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

H. Peter Anvin
2007-11-29T18:47:43Z

Linus Torvalds wrote:
>
> However, you also say:
>
>> 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.

-hpa