Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752021AbdHQIbR (ORCPT ); Thu, 17 Aug 2017 04:31:17 -0400 Received: from mail-oi0-f68.google.com ([209.85.218.68]:37995 "EHLO mail-oi0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750978AbdHQIbP (ORCPT ); Thu, 17 Aug 2017 04:31:15 -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 16:31:14 +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 v7H8VNSd020528 Content-Length: 3153 Lines: 69 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