From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: KVM and OS/2: #UD in real mode Date: Wed, 23 Jan 2008 22:40:03 -0800 Message-ID: <479832C3.1040004@zytor.com> References: <4796AB3F.5070407@zytor.com> <4798312D.9060204@qumranet.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel To: Avi Kivity Return-path: In-Reply-To: <4798312D.9060204-atKUWr5tajBWk0Htik3J/w@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org Errors-To: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org List-Id: kvm.vger.kernel.org Avi Kivity wrote: > H. Peter Anvin wrote: >> Just for fun, I tried to boot OS/2 Warp 4.0 under KVM (KVM-59 with the >> latest git kernel from Linus as of yesterday, slightly post >> 2.6.24-rc8.) I found that it crashes very early, apparently because >> KVM doesn't handle an #UD received in user mode. It appears that OS/2 >> actually provokes an #UD deliberately in real mode, from the >> disassembly it looks like it's trying to probe for the 486 version of >> cmpxchg (which has a different opcode than the 586+ version.) >> > > Strange, the manual lists 0f b0 and 0f b1 as compatible all the way back > to the 486. What opcode are you seeing? > 0F A6. Was apparently used on A-step 486's. Could also just be a nonsense opcode used for trapping, or something like that. >> It looks like the kernel code filters out a very small number of >> real-mode exceptions, and does a KVM exit for all the other ones; the >> userspace code then unconditionally barfs. This is presumably a >> temporary hack; what is the intended behaviour - for this to be >> handled in-kernel, or in userspace? > > In kernel. I've never seen a #UD in real mode, that's why it isn't > handled. > Just was a bit surprised to note that it explicitly tests for a small number of traps, when all the traps can be taken in real mode (via the IVT.) -hpa ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/