[parisc-linux] Generic light-weight syscall.

[Matthew Wilcox]

On Mon, Jul 28, 2003 at 01:30:41PM -0700, Randolph Chung wrote:
> Why not add a flag to syscall() which indicates whether this is a "fast"
> syscall or a "slow" syscall, and based on this, decide whether to do all
> the register spilling, etc when entering the kernel?  Then we can
> implement the atomic ops as additional "syscalls"....
> 
> I would think that there is at least some amount of logic that needs to
> be there everytime you enter/exit the kernel, irregardless of whether
> you are doing a "fast syscall" (i.e. no need to save the processor
> state, etc) or a regular one... i would hope we don't need to have two
> copies of that logic.
> 
> Carlos had some concerns that this means fast syscalls (or regular ones
> perhaps) will always incur a mispredicted branch and/or extra stack
> manipulations that may not be needed..... but i'm not yet convinced that
> there is enough overhead to make this a problem. what do others think?

I don't think enough syscalls are "fast" to make this worth doing.
Most of the things people are talking about for lightweight syscalls are
things that could/should be done in userspace ... except that you need to
be privileged to use them.  So the optimum way to solve them is to have a
special calling convention and todo the absolute minimum amount of work.
Setting cr26 is a great example because we would do it in userspace if
the architecture let us.

Does anyone have a better definition for lightweight syscall?  How about
"Cannot sleep, called frequently, must be fast"?

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