From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1JtuAn-0006GC-DJ
	for qemu-devel@nongnu.org; Wed, 07 May 2008 20:39:49 -0400
Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43)
	id 1JtuAl-0006F8-8K
	for qemu-devel@nongnu.org; Wed, 07 May 2008 20:39:48 -0400
Received: from [199.232.76.173] (port=56117 helo=monty-python.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1JtuAk-0006F0-UH
	for qemu-devel@nongnu.org; Wed, 07 May 2008 20:39:47 -0400
Received: from mail.codesourcery.com ([65.74.133.4]:51774)
	by monty-python.gnu.org with esmtps
	(TLS-1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.60)
	(envelope-from <paul@codesourcery.com>) id 1JtuAk-0006bK-8i
	for qemu-devel@nongnu.org; Wed, 07 May 2008 20:39:46 -0400
From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] Re: LSI: avoid infinite loops
Date: Thu, 8 May 2008 01:39:38 +0100
References: <20080507230206.GB28197@dmt>
	<200805080021.55113.paul@codesourcery.com>
	<20080507234239.GA28545@dmt>
In-Reply-To: <20080507234239.GA28545@dmt>
MIME-Version: 1.0
Content-Type: text/plain;
  charset="ansi_x3.4-1968"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
Message-Id: <200805080139.39137.paul@codesourcery.com>
Reply-To: qemu-devel@nongnu.org
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/pipermail/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: qemu-devel@nongnu.org
Cc: Alberto =?utf-8?q?Trevi=C3=B1o?= <alberto@byu.edu>, Marcelo Tosatti <mtosatti@redhat.com>

On Thursday 08 May 2008, Marcelo Tosatti wrote:
> On Thu, May 08, 2008 at 12:21:54AM +0100, Paul Brook wrote:
> > On Thursday 08 May 2008, Marcelo Tosatti wrote:
> > > The Windows driver has SCRIPTS code which busy loops on main memory. So
> > > give the CPU's a chance to run if that happens.
> >
> > I'm kinda surprised this works.  What causes the scripts engine to be
> > restarted?
>
> LSI_ISTAT0_SIGP.

In that case my surprise continues, and this is looking like an unbelievably 
horrid hack.

By my reading you're making LSI_ISTAT0_SIGP effect whatever instruction 
happens to be executing when we stall. You get doubly lucky because (a) the 
guest OS decides to bang on SIGP, even though it doesn't need to. And (b) the 
last instruction executed happens to have set dnad to a value that "works". 
I'm guessing you always happen to stop execution on the conditional jump 
instruction and taking that jump doesn't cause any bad effects, right?

Paul