Re: [PATCH 1/5] Split wait_noreap_copyout()

[Vitaly Mayatskikh]

At Mon, 11 May 2009 14:17:08 +0200, Ingo Molnar wrote:

> > 	if (put_user(signal, &infop->si_signo) ||
> > 	    put_user(0, &infop->si_errno) ||
> > 	    put_user((short)why, &infop->si_code) ||
> > 	    put_user(pid, &infop->si_pid) ||
> > 	    put_user(uid, &infop->si_uid) ||
> > 	    put_user(status, &infop->si_status))
> > 		return -EFAULT;
> 
> For best assembly code this should generally be written as a series 
> of:
> 
>    __uaccess_err |= __put_user(x, ptr);
>    __uaccess_err |= __put_user(y, ptr);
>    __uaccess_err |= __put_user(z, ptr);
> 
> As this generates non-dependent, compressed, branch-less code.

Yeah, my first intention was to eliminate a lot of branches in one
place. It's terrible for CPU pipeline, I bet.

> See the (new) put_user_try / put_user_ex() / put_user_catch() 
> abstraction in arch/x86/include/asm/uaccess.h, and how all the x86 
> signal code makes use of that to optimize such patterns of per field 
> user copies.

So, there's catch block to handle GPF and the code inside of `try'
block is still branch-less, right? I was thinking of minimized version
of struct siginfo (up to si_uid) and copying it with single
copy_to_user(), but the idea with try/catch is definitely much
better.

-- 
wbr, Vitaly
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