Fast syscalls via sysenter

[Konstantin Belousov]

On Sat, Jun 23, 2012 at 02:17:53PM +0800, David Xu wrote:
> 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

Yes, intent is to eventually switch to VDSO from current situation were
libc is aware of shared page content. This was extensively discussed in
flame that resulted in me writing the current gettimeofday(2) patch.
It was arch@ several weeks ago, AFAIR.

Committed gettimeofday() code structure allows for VDSO interposing without
breaking normal symbol visibility rules.

I do not see a sense in implementing syscall or sysenter support for
i386 kernel. On the other hand, using syscall for 32bit binaries on amd64
looks reasonable.
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