Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751848AbdHQIvp (ORCPT ); Thu, 17 Aug 2017 04:51:45 -0400 Received: from mail-oi0-f68.google.com ([209.85.218.68]:36333 "EHLO mail-oi0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751167AbdHQIvn (ORCPT ); Thu, 17 Aug 2017 04:51:43 -0400 MIME-Version: 1.0 In-Reply-To: References: <1502890494-35208-1-git-send-email-pbonzini@redhat.com> <20170816170843-mutt-send-email-mst@kernel.org> <20170816165625.GA32542@flask> <1649ef9d-1d78-235d-bb14-ca1949ffc0e9@gmail.com> From: Wanpeng Li Date: Thu, 17 Aug 2017 16:51:42 +0800 Message-ID: Subject: Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO To: Yang Zhang Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , "Michael S. Tsirkin" , Paolo Bonzini , "linux-kernel@vger.kernel.org" , kvm , "# v3 . 10+" , Jason Wang Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by nfs id v7H8ppiY024004 Content-Length: 4000 Lines: 103 2017-08-17 16:48 GMT+08:00 Yang Zhang : > On 2017/8/17 16:31, Wanpeng Li wrote: >> >> 2017-08-17 16:28 GMT+08:00 Wanpeng Li : >>> >>> 2017-08-17 16:07 GMT+08:00 Yang Zhang : >>>> >>>> On 2017/8/17 0:56, Radim Krčmář wrote: >>>>> >>>>> >>>>> 2017-08-16 17:10+0300, Michael S. Tsirkin: >>>>>> >>>>>> >>>>>> On Wed, Aug 16, 2017 at 03:34:54PM +0200, Paolo Bonzini wrote: >>>>>>> >>>>>>> >>>>>>> Microsoft pointed out privately to me that KVM's handling of >>>>>>> KVM_FAST_MMIO_BUS is invalid. Using skip_emulation_instruction is >>>>>>> invalid >>>>>>> in EPT misconfiguration vmexit handlers, because neither EPT >>>>>>> violations >>>>>>> nor misconfigurations are listed in the manual among the VM exits >>>>>>> that >>>>>>> set the VM-exit instruction length field. >>>>>>> >>>>>>> While physical processors seem to set the field, this is not >>>>>>> architectural >>>>>>> and is just a side effect of the implementation. I couldn't convince >>>>>>> myself of any condition on the exit qualification where VM-exit >>>>>>> instruction length "has" to be defined; there are no trap-like >>>>>>> VM-exits >>>>>>> that can be repurposed; and fault-like VM-exits such as >>>>>>> descriptor-table >>>>>>> exits provide no decoding information. So I don't really see any way >>>>>>> to keep the full speedup. >>>>>>> >>>>>>> What we can do is use EMULTYPE_SKIP; it only saves 200 clock cycles >>>>>>> because computing the physical RIP and reading the instruction is >>>>>>> expensive, but at least the eventfd is signaled before entering the >>>>>>> emulator. This saves on latency. While at it, don't check >>>>>>> breakpoints >>>>>>> when skipping the instruction, as presumably any side effect has been >>>>>>> exposed already. >>>>>>> >>>>>>> Adding a hypercall or MSR write that does a fast MMIO write to a >>>>>>> physical >>>>>>> address would do it, but it adds hypervisor knowledge in virtio, >>>>>>> including >>>>>>> CPUID handling. So it would be pretty ugly in the guest-side >>>>>>> implementation, >>>>>>> but if somebody wants to do it and the virtio side is acceptable to >>>>>>> the >>>>>>> virtio maintainers, I am okay with it. >>>>>>> >>>>>>> Cc: Michael S. Tsirkin >>>>>>> Cc: stable@vger.kernel.org >>>>>>> Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae >>>>>>> Suggested-by: Radim Krčmář >>>>>>> Signed-off-by: Paolo Bonzini >>>>>> >>>>>> >>>>>> >>>>>> Jason (cc) who worked on the original optimization said he can >>>>>> work to test the performance impact. >>>>>> I suggest we don't rush this (it's been like this for 2 years), >>>>>> and the issue seems to be largely theoretical. >>>>> >>>>> >>>>> >>>>> Paolo, did Microsoft point it out because they hit the bug when running >>>>> KVM on Hyper-V? >>>> >>>> >>>> >>>> Does this mean the nested emulation of EPT violation and >>>> misconfiguration in >>>> KVM side doesn't strictly follow the manual since we didn't hit the bug >>>> in >>>> KVM? >>> >>> >>> The VM-exit instruction length of vmcs12 is provided by vmcs02 >>> (prepare_vmcs12()), so unless the length from vmcs02 is wrong. In >>> addition, something like mov instruction which can trigger the EPT >>> violation/misconfig in guest has already been decoded before executing >>> I think, IIUC, then exit qualification can have the information about >>> the instruction length. >> >> >> s/exit qualification/VM-exit instruction length > > > According to Paolo's comment "neither EPT violations nor misconfigurations > are listed in the manual among the VM exits that set the VM-exit instruction > length field", it seems to set the instruction length in vmcs12 is not right > though it is harmless. But Paolo also mentioned this "It just happens that the actual condition for VM-exit instruction length being set correctly is "the fault was taken after the accessing instruction has been decoded"." Regards, Wanpeng Li