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: Wed, 17 Feb 2010 16:13:11 +0100 Message-ID: <4B7C0787.30405@siemens.com> References: <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> <20100217105527.GQ2995@redhat.com> <4B7BD3B5.2080207@siemens.com> <20100217130325.GU2995@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Avi Kivity , Marcelo Tosatti , kvm To: Gleb Natapov Return-path: Received: from david.siemens.de ([192.35.17.14]:22394 "EHLO david.siemens.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757346Ab0BQPNb (ORCPT ); Wed, 17 Feb 2010 10:13:31 -0500 In-Reply-To: <20100217130325.GU2995@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: Gleb Natapov wrote: > On Wed, Feb 17, 2010 at 12:32:05PM +0100, Jan Kiszka wrote: >> Gleb Natapov wrote: >>> 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. >>>> >>> I tested it on processor without NextRIP and your test case works there too, >>> but it shouldn't have, so I looked deeper into that and what I see is >>> that GDB outsmart us. It doesn't matter if we inject event before int3 >>> inserted by GDB or after it GDB correctly finds breakpoint that >>> triggered and restart instruction correctly. I assume it doesn't use >>> exact match between rip where int3 was inserted and where exceptions >>> triggers. >> At latest when you have two successive breakpoints on single-byte >> instructions, gdb will reach its limits (for it failed earlier, BTW). >> And other debuggers under other OSes may become unhappy as well. > Yes, and that is why I am saying checking with GDB is not a good test. > GDB may work, but it doesn't mean injection works correctly. It took me > some time to write test that finally confused gdb. It was like this: > > 1: int main(int argc, char **argv) > 2: { > 3: if (argc == 1) > 4: goto a; > 5: asm("cmc"); > 6: a: > 7: asm("cmc"); > 8: return 0; > 9: } > > If you set breakpoint on lines 5 and 7 when breakpoint triggers GDB > thinks it is on line 5. > > So can you run int3 test below on master on AMD with NextRIP support? > I doubt the result will be correct. If you meant your test above: Works out of the box with unpatched kvm on modern AMD CPUs, ie. gdb always stops at line 7 even if host debugging is active. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux