From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: [PATCH] KVM: VMX: Update instruction length on intercepted BP Date: Mon, 15 Feb 2010 15:30:42 +0200 Message-ID: <20100215133042.GC19478@redhat.com> References: <4B77D4DE.3030602@web.de> <20100214111544.GJ2511@redhat.com> <4B77E0E2.7030704@web.de> <20100214144501.GN2511@redhat.com> <4B7826D3.7080201@web.de> <20100214165319.GA19246@redhat.com> <4B782D97.9030304@web.de> <20100214172613.GB19246@redhat.com> <4B7837A3.4040607@web.de> <4B794A1F.7050009@siemens.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Avi Kivity , Marcelo Tosatti , kvm To: Jan Kiszka Return-path: Received: from mx1.redhat.com ([209.132.183.28]:55698 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751034Ab0BONaz (ORCPT ); Mon, 15 Feb 2010 08:30:55 -0500 Content-Disposition: inline In-Reply-To: <4B794A1F.7050009@siemens.com> Sender: kvm-owner@vger.kernel.org List-ID: On Mon, Feb 15, 2010 at 02:20:31PM +0100, Jan Kiszka wrote: > Jan Kiszka wrote: > > Gleb Natapov wrote: > >> Lets check if SVM works. I can do that if you tell me how. > > > > - Fire up some Linux guest with gdb installed > > - Attach gdb to gdbstub of the VM > > - Set a soft breakpoint in guest kernel, ideally where it does not > > immediately trigger, e.g. on sys_reboot (use grep sys_reboot > > /proc/kallsyms if you don't have symbols for the guest kernel) > > - Start gdb /bin/true in the guest > > - run > > > > As gdb sets some automatic breakpoints, this already exercises the > > reinjection of #BP. > > I just did this on our primary AMD platform (Embedded Opteron, 13KS EE), > and it just worked. > > But this is a fairly new processor. Consequently, it reports NextRIP > support via cpuid function 0x8000000A. Looking for an older one too. > > In the meantime I also browsed a bit more in the manuals, and I don't > think stepping over or (what is actually required) into an INT3 will > work. We can't step into as the processor clears TF on any event handler > entry. And stepping over would cause troubles > > a) as an unknown amount of code may run without #DB interception > b) we would fiddle with TF in code that is already under debugger > control, thus we would very likely run into conflicts. > > Leaves us with tricky INT3 emulation. Sigh. > So the question is do we want to support this kind of debugging on older AMDs. May we don't. Then lets apply your patch for VMX with a comment that explains why we need to save instruction length here (int3 will be reinjected from userspace). -- Gleb.