Hi!

I am trying to continue the work started by DavidXu on implemention of fast
syscalls via sysenter/sysexit.
http://people.freebsd.org/~davidxu/sysenter/kernel/
I have ported it on FreeBSD9. It looks like it works. Unfortunately I am a
beginner in kernel so I have some questions:

1. see http://people.freebsd.org/~davidxu/sysenter/kernel/kernel.patch
/*
* If %edx was changed, we can not use sysexit, because it
* needs %edx to restore userland %eip.
*/
if (orig_edx != frame.tf_edx)
td->td_pcb->pcb_flags |= PCB_FULLCTX;

What is the reason why we have to do this additional check? In
http://people.freebsd.org/~davidxu/sysenter/kernel/sysenter.s
we store %edx to the stack in
pushl %edx /* ring 3 next %eip */
and we restore the register in
popl %edx /* ring 3 %eip */

2. see http://people.freebsd.org/~davidxu/sysenter/kernel/sysenter.s
movl PCPU(CURPCB),%esi
call syscall

Why do we  movl PCPU(CURPCB),%esi before calling syscall? syscall is just c-
function.


--
Daniil Cherednik

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