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

["H. Peter Anvin" <hpa@zytor.com>]

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