Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752066AbdHQIHf (ORCPT ); Thu, 17 Aug 2017 04:07:35 -0400 Received: from mail-oi0-f67.google.com ([209.85.218.67]:36012 "EHLO mail-oi0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751848AbdHQIHY (ORCPT ); Thu, 17 Aug 2017 04:07:24 -0400 Subject: Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , "Michael S. Tsirkin" References: <1502890494-35208-1-git-send-email-pbonzini@redhat.com> <20170816170843-mutt-send-email-mst@kernel.org> <20170816165625.GA32542@flask> Cc: Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, stable@vger.kernel.org, Jason Wang From: Yang Zhang Message-ID: <1649ef9d-1d78-235d-bb14-ca1949ffc0e9@gmail.com> Date: Thu, 17 Aug 2017 16:07:17 +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: <20170816165625.GA32542@flask> 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: 2498 Lines: 56 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? > > Thanks. > -- Yang Alibaba Cloud Computing