Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751572AbdHQItF (ORCPT ); Thu, 17 Aug 2017 04:49:05 -0400 Received: from mail-oi0-f65.google.com ([209.85.218.65]:34100 "EHLO mail-oi0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750755AbdHQItB (ORCPT ); Thu, 17 Aug 2017 04:49:01 -0400 Subject: Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO To: Wanpeng Li 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> Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , "Michael S. Tsirkin" , Paolo Bonzini , "linux-kernel@vger.kernel.org" , kvm , "# v3 . 10+" , Jason Wang From: Yang Zhang Message-ID: Date: Thu, 17 Aug 2017 16:48:55 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3552 Lines: 79 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. -- Yang Alibaba Cloud Computing