linux-kernel - Re: [patch 0/6] sys_indirect RFC

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Date:	Mon, 2 Jul 2007 15:37:01 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Ulrich Drepper <drepper@...il.com>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 0/6] sys_indirect RFC - sys_indirect introduction

On Mon, 2 Jul 2007, Ulrich Drepper wrote:

> On 7/1/07, Davide Libenzi <davidel@...ilserver.org> wrote:
> > With the current API design you'd able to easily confine the "pre" code
> > inside the "set" function, and the "post" code inside the "unset"
> > function. It looks pretty clean to me, and allows to limit the knowledge
> > of sys_indirect, the more as possible inside kernel/indirect.c.
> 
> But this will not be applicable.  We already discussed that each
> syscall likely needs its own set of flags etc.  There really isn't
> much overlap if any which cannot be handled at least as well using a
> flat structure.  You're adding major complications for something which
> IMO will never be usable.  With the flat structure to whole overhead
> of sys_indirect is limited to a test for valid syscalls, copying the
> struct, making the call to the syscall function, and resetting the
> value in current.  Very simple and fast.

Never be usable? I made you a concrete example that is like 8 months old. 
And *that* could not have been cleanly handled with the flat structure 
idea.
The extra flags parameter is one example where we'd need an extra flags 
field in the task_struct in any case. So you need in any case code that 
does extra checks and merges normal parameters/flags with the shared 
context ones. This independently of the method used. But there are 
examples (and the signal stuff is one of them), where you do need the 
set_context+syscall+unset_context abstraction, for all cases where the 
kernel already has its own internal data strctures. In those cases you'd 
have to spread sys_internal context knowledge all around the kernel, 
whereas the current solution allows you to confine the code inside 
kernel/indirect.c (through the set/unset abstraction). And this w/out even 
try to hit the weak spot of about how this structure will look after a few 
additions.

- Davide

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