Fast syscalls via sysenter
John Baldwin
jhb at freebsd.org
Thu Jun 21 12:22:07 UTC 2012
On Monday, June 18, 2012 2:56:30 pm Daniil Cherednik wrote:
> 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 */
Some system calls return two return values (pipe(2)) or return a 64-bit
off_t (lseek(2)). Those system calls change %edx's value and need that
changed value to make it out to userland.
> 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.
No clue on this one, looks like it is not needed.
--
John Baldwin
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