Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965167AbVKGRgM (ORCPT ); Mon, 7 Nov 2005 12:36:12 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S964994AbVKGRgL (ORCPT ); Mon, 7 Nov 2005 12:36:11 -0500 Received: from mailout1.vmware.com ([65.113.40.130]:38674 "EHLO mailout1.vmware.com") by vger.kernel.org with ESMTP id S965098AbVKGRgH (ORCPT ); Mon, 7 Nov 2005 12:36:07 -0500 Message-ID: <436F8FAE.90805@vmware.com> Date: Mon, 07 Nov 2005 09:32:30 -0800 From: Zachary Amsden User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.2) Gecko/20040803 X-Accept-Language: en-us, en MIME-Version: 1.0 To: "linux-os (Dick Johnson)" Cc: "Maciej W. Rozycki" , Andi Kleen , linux-kernel@vger.kernel.org Subject: Re: 2.6.14: CR4 not needed to be inspected on the 486 anymore? References: <436A3C10.9050302@vmware.com> <436AA1FD.3010401@vmware.com> <436F7673.5040309@vmware.com> <436F8601.4070201@vmware.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 07 Nov 2005 17:32:31.0655 (UTC) FILETIME=[3B82D370:01C5E3C1] Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1822 Lines: 62 linux-os (Dick Johnson) wrote: >On Mon, 7 Nov 2005, Zachary Amsden wrote: > > > >>Maciej W. Rozycki wrote: >> >> >> >>>On Mon, 7 Nov 2005, Zachary Amsden wrote: >>> >>> >>> >>> >>> >>>>While this is at least no worse in the nested fault case than earlier >>>>kernels, I really wish I had one of those weird 486s so I could test the >>>>faulting mechanism. It seems the trap handling code has gotten quite >>>> >>>> >>>> >>>> >>>What's so weird about 486s? Besides, for testing it doesn't have to be >>>one -- you will get away with a 386, too. I have neither anymore, but >>>there are people around still using them. >>> >>> >>> >>> >>Because I hold in my hand "i486 Microprocessor Programmer's Reference >>Manual, c 1990", and it has no mention whatsoever of CR4, and all >>documentation I had until Friday had either no mention of CR4, or >>something to the effect of "new on Pentium, the CR4 register ..." So >>I've had to re-adjust my definition of 486, which was weird. >> >>Zach >>- >> >> > >Yes, and undocumented opcodes might not fault. They might do nothing >or something strange. It's not a good idea to use an undocumented >opcode in kernel space. The read-from-CR4 in kernel space, hoping >that an immoral-opcode trap will save you is not good practice. > >You might reset the processor. > > No, you won't. #UD and #GP will not (I hesitate to say never, but other than a processor bug, I believe that is correct) reset the processor. And CR4 is not "undocumented", even on 486. What is immoral about opcode trapping? Zach - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/