On 2012/06/21 20:11, John Baldwin wrote:
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.
[kib@ is cc'ed]
I implemented the sysenter syscall long time ago, it indeed can reduce
system call overhead on i386. I think it might be the time to implement
linux like vdso syscall now based on the work kib@ recently has done,
though I don''t know how to hook it into kib's code.
I quick googled it, and found they put some data into aux vector:
http://www.trilithium.com/johan/2005/08/linux-gate/
http://www.takatan.net/lxr/source/arch/um/os-Linux/elf_aux.c?a=x86_64#L40
Regards,
David Xu
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