From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [PATCH] KVM: VMX: Update instruction length on intercepted BP Date: Sun, 14 Feb 2010 18:49:23 +0100 Message-ID: <4B7837A3.4040607@web.de> References: <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> <20100214165319.GA19246@redhat.com> <4B782D97.9030304@web.de> <20100214172613.GB19246@redhat.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enig98C344CF890BA932E6D20B15" Cc: Avi Kivity , Marcelo Tosatti , kvm To: Gleb Natapov Return-path: Received: from fmmailgate01.web.de ([217.72.192.221]:45999 "EHLO fmmailgate01.web.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752417Ab0BNRuS (ORCPT ); Sun, 14 Feb 2010 12:50:18 -0500 In-Reply-To: <20100214172613.GB19246@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig98C344CF890BA932E6D20B15 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Gleb Natapov wrote: > On Sun, Feb 14, 2010 at 06:06:31PM +0100, Jan Kiszka wrote: >> Gleb Natapov wrote: >>> 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 explici= tly in such >>>>>>>>>>>> cases. At least in the absence of migration, this ensures th= at >>>>>>>>>>>> re-injections of #BP will find and use the correct instructi= on length. >>>>>>>>>>>> >>>>>>>>>>> event_exit_inst_len is only used for event reinjection. Since= event >>>>>>>>>>> intercepted here will not be reinjected why updating event_ex= it_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 ex= ception 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 t= hat 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 f= rom 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 th= at 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 th= e int3 >>>>>>>> when requesting to inject #BP. >>>>>>>> >>>>>>> This will work for VMX too, no? So may be we should design someth= ing >>>>>>> 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 o= f VMX. >>>>>> >>>>> VMX used to always reexecute instruction. >>>> ...since 66fd3f7f90. And that was what broke this guest debugging co= rner >>>> case. >>>> >>> I see. And I see why it worked, but it shouldn't have been working fo= r >>> SVM. I prefer to look for general solution here that works for SVM/VM= X. >> I don't see the need to emulate INT3 for the sake of unification. VMX >> works today (with this patch), and SVM might work without further >> efforts, at least on modern hosts: >> >> "Software interrupts cannot be properly injected if the processor does= >> not support the NextRIP field, indicated by EDX[3] =3D 1 as returned b= y >> CPUID function 8000_000A. Hypervisor software should emulate the event= >> injection of software interrupts if NextRIP is not supported." >> >> (right below the paragraph I cited before) >> >> I assume, INT3 can be considered as software interrupt as well in this= >> context. >> > 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. > My concern is > that if SVM doesn't work we will write another way to handle this > situation that will work on SVM and VMX and then we will have code in > VMX that is not needed, but we have to support it. I understand your concerns, but I do not share them ATM. I bet we will need some workaround for older SVM, but it will be some SVM-only thing. Anyway, we can wait with this patch a few days until we know more. Jan --------------enig98C344CF890BA932E6D20B15 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (GNU/Linux) Comment: Using GnuPG with SUSE - http://enigmail.mozdev.org iEYEARECAAYFAkt4N6YACgkQitSsb3rl5xTU1ACg3HDxfNkEdAevlztTw7oJkSvU VQIAoL+JXxoT2VnpWyj0YFMrFsSii/fj =Rc66 -----END PGP SIGNATURE----- --------------enig98C344CF890BA932E6D20B15--