Re: Intel P6 vs P7 system call performance

[Pavel Machek]

Hi!

> > > But this is exactly what I expect to happen.  If you want to implement
> > > gettimeofday() at user-level you need to modify the page.
> > 
> > Note that I really don't think we ever want to do the user-level
> > gettimeofday(). The complexity just argues against it, it's better to try
> > to make system calls be cheap enough that you really don't care.
> 
> I'd say that this should not be "fixed" from userspace, but from the
> kernel. Thus if the kernel knows that the "gettimeofday" can be made
> faster by doing it completely in userspace, then that system call
> should be "patched" by the kernel to do it faster for everybody.
> 
> Next, someone might find a faster (full-userspace) way to do some
> "reads"(*). Then it might pay to check for that specific
> filedescriptor in userspace, and only call into the kernel for the
> other filedescriptors. The idea is that the kernel knows best when
> optimizations are possible.
> 
> Thus that ONE magic address is IMHO not the right way to do it. By
> demultiplexing the stuff in userspace, you can do "sane" things with
> specific syscalls. 
> 
> So for example, the code at 0xffff80000 would be: 
> 	mov 0x00,%eax
> 	int $80
> 	ret
> 
> (in the case where sysenter & friends is not available)

This could save that one register needed for 6-args syscalls. If code
at 0xffff8000 was mov %ebp, %eax; sysenter; ret for P4, you could do
6-args syscalls this way.
								Pavel
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