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

From: H. Peter Anvin
Date: Thu Nov 29 2007 - 13:47:45 EST


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
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