Re: [PATCH 01/23] all: syscall wrappers: add documentation

[Catalin Marinas]

On Fri, May 27, 2016 at 08:03:57AM +0200, Heiko Carstens wrote:
> > > > The cost is pretty trivial though. See kernel/compat_wrapper.o:
> > > > COMPAT_SYSCALL_WRAP2(creat, const char __user *, pathname, umode_t, mode);
> > > > 0:   a9bf7bfd        stp     x29, x30, [sp,#-16]!
> > > > 4:   910003fd        mov     x29, sp
> > > > 8:   2a0003e0        mov     w0, w0
> > > > c:   94000000        bl      0 <sys_creat>
> > > > 10:  a8c17bfd        ldp     x29, x30, [sp],#16
> > > > 14:  d65f03c0        ret
> > > 
> > > I would say the above could be more expensive than 8 movs (16 bytes to
> > > write, read, a branch and a ret). You can also add the I-cache locality,
> > > having wrappers for each syscalls instead of a single place for zeroing
> > > the upper half (where no other wrapper is necessary).
> > > 
> > > Can we trick the compiler into doing a tail call optimisation. This
> > > could have simply been:
> > > 
> > > COMPAT_SYSCALL_WRAP2(creat, ...):
> > > 	mov	w0, w0
> > > 	b	<sys_creat>
> > 
> > What you talk about was in my initial version. But Heiko insisted on having all
> > wrappers together.
> > http://www.spinics.net/lists/linux-s390/msg11593.html
> > 
> > Grep your email for discussion.
> 
> I think Catalin's question was more about why there is even a stack frame
> generated. It looks like it is not necessary. I did ask this too a couple
> of months ago, when we discussed this.

Indeed, I was questioning the need for prologue/epilogue, not the use of
COMPAT_SYSCALL_WRAPx(). Maybe something like __naked would do.

> > > > > Cost wise, this seems like it all cancels out in the end, but what
> > > > > do I know?
> > > > 
> > > > I think you know something, and I also think Heiko and other s390 guys
> > > > know something as well. So I'd like to listen their arguments here.
> 
> If it comes to 64 bit arguments for compat system calls: s390 also has an
> x32-like ABI extension which allows user space to use full 64 bit
> registers. As far as I know hardly anybody ever made use of that.
> 
> However even if that would be widely used, to me it wouldn't make sense to
> add new compat system calls which allow 64 bit arguments, simply because
> something like
> 
> c = (u32)a | (u64)b << 32;
> 
> can be done with a single 1-cycle instruction. It's just not worth the
> extra effort to maintain additional system call variants.

If we split 64-bit arguments in two, we can go a step further and avoid
most of the COMPAT_SYSCALL_WRAPx annotations in favour of a common
upper-half zeroing of the argument registers on ILP32 syscall entry.
There would be a few exceptions where we need to re-build 64-bit
arguments on sign-extend.

-- 
Catalin