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: Sun, 14 Feb 2010 18:53:19 +0200 Message-ID: <20100214165319.GA19246@redhat.com> References: <4B767160.4070609@web.de> <20100214075303.GF2511@redhat.com> <4B77CFD7.9080504@web.de> <20100214103445.GH2511@redhat.com> <4B77D4DE.3030602@web.de> <20100214111544.GJ2511@redhat.com> <4B77E0E2.7030704@web.de> <20100214144501.GN2511@redhat.com> <4B7826D3.7080201@web.de> 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]:3309 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751908Ab0BNQxV (ORCPT ); Sun, 14 Feb 2010 11:53:21 -0500 Content-Disposition: inline In-Reply-To: <4B7826D3.7080201@web.de> Sender: kvm-owner@vger.kernel.org List-ID: On Sun, Feb 14, 2010 at 05:37:39PM +0100, Jan Kiszka wrote: > Gleb Natapov wrote: > > On Sun, Feb 14, 2010 at 12:39:14PM +0100, Jan Kiszka wrote: > >> Gleb Natapov wrote: > >>> On Sun, Feb 14, 2010 at 11:47:58AM +0100, Jan Kiszka wrote: > >>>> Gleb Natapov wrote: > >>>>> On Sun, Feb 14, 2010 at 11:26:31AM +0100, Jan Kiszka wrote: > >>>>>> Gleb Natapov wrote: > >>>>>>> On Sat, Feb 13, 2010 at 10:31:12AM +0100, Jan Kiszka wrote: > >>>>>>>> From: Jan Kiszka > >>>>>>>> > >>>>>>>> We intercept #BP while in guest debugging mode. As VM exists due to > >>>>>>>> intercepted exceptions do not necessarily come with valid > >>>>>>>> idt_vectoring, we have to update event_exit_inst_len explicitly in such > >>>>>>>> cases. At least in the absence of migration, this ensures that > >>>>>>>> re-injections of #BP will find and use the correct instruction length. > >>>>>>>> > >>>>>>> event_exit_inst_len is only used for event reinjection. Since event > >>>>>>> intercepted here will not be reinjected why updating event_exit_inst_len > >>>>>>> is needed here? > >>>>>> In guest debugging mode a #BP exception is always reported to user space > >>>>>> to find out what caused it. If it was the guest itself, the exception is > >>>>>> reinjected, on older kernels via KVM_SET_GUEST_DEBUG and since 2.6.33 > >>>>>> via KVM_SET_VCPU_EVENTS (the latter requires some qemu patch that I will > >>>>>> post later). > >>>>>> > >>>>>> As we currently do not update event_exit_inst_len on #BP exits, > >>>>>> reinjecting fails unless event_exit_inst_len happens to be 1 from some > >>>>>> other exit. > >>>>>> > >>>>> Hmm, how does it work on SVM then where we do not have > >>>>> event_exit_inst_len so execution will resume on the same rip that caused > >>>>> #BP after event reinjection? > >>>>> > >>>> Maybe not at all. I don't think I've tested this scenario on amd so far. > >>>> Guess it needs some special handling in svm to move rip after the int3 > >>>> when requesting to inject #BP. > >>>> > >>> This will work for VMX too, no? So may be we should design something > >>> that will work for both VMX and SVM before applying patches that make > >>> oly VMX work? > >> VMX used to work, so my patch is actually a regression fix. I bet this > >> was accidentally broken while cleaning up the interrupt handling of VMX. > >> > > VMX used to always reexecute instruction. > > ...since 66fd3f7f90. And that was what broke this guest debugging corner > case. > I see. And I see why it worked, but it shouldn't have been working for SVM. I prefer to look for general solution here that works for SVM/VMX. -- Gleb.