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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