linux-kernel - Re: [PATCH 0/3] clone64() and unshare64() system calls

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Date:	Thu, 10 Apr 2008 11:31:41 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	sukadev@...ibm.com
CC:	Paul Menage <menage@...gle.com>, Andrew Morton <akpm@...l.org>,
	clg@...ibm.com, serue@...ibm.com,
	"David C. Hansen" <haveblue@...ibm.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Containers <containers@...ts.osdl.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] clone64() and unshare64() system calls

sukadev@...ibm.com wrote:
> | 
> | I thought that the consensus was that adding a new system call was
> | better than trying to force extensibility on to the existing
> | non-extensible system call.
> 
> There were couple of objections to extensible system calls like
> sys_indirect() and to Pavel's approach.
> 

This is a very different thing, though.  sys_indirect is pretty much a 
mechanism for having a sideband channel -- a second ABI -- into each and 
every system call, making it extremely hard to analyze what the full set 
of impact of a specific system call is.  Worse, as it was being proposed 
to have been used, it would have set state variables inside the kernel 
in a very opaque manner.

> | But if we are adding a new system call, why not make the new one
> | extensible to reduce the need for yet another new call in the future?
> 
> hypothetically, can we make a variant of clone() extensible to the point
> of requiring a copy_from_user() ?

The only issue is whether or not it's acceptable from a performance 
standpoint.  clone() is reasonably expensive, though.

	-hpa
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