Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752071AbdHRIsj (ORCPT ); Fri, 18 Aug 2017 04:48:39 -0400 Received: from mx1.redhat.com ([209.132.183.28]:43904 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750775AbdHRIqu (ORCPT ); Fri, 18 Aug 2017 04:46:50 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com D69C3C0587E8 Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx08.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=jasowang@redhat.com Subject: Re: [PATCH] kvm: VMX: do not use vm-exit instruction length for fast MMIO To: "Michael S. Tsirkin" , Paolo Bonzini Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, rkrcmar@redhat.com, stable@vger.kernel.org References: <1502890494-35208-1-git-send-email-pbonzini@redhat.com> <20170816170843-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: Date: Fri, 18 Aug 2017 16:46:38 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170816170843-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Fri, 18 Aug 2017 08:46:50 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2414 Lines: 54 On 2017年08月16日 22:10, Michael S. Tsirkin wrote: > 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 see regressions on both latency and cpu utilization through netperf TCP_RR test: pkt_size/sessions/+transaction_rate%/+per_cpu_transaction_rate% 1/ 1/ +0%/ -5% 1/ 25/ -1%/ -2% 1/ 50/ -9%/ -10% 64/ 1/ -3%/ -9% 64/ 25/ 0%/ -2% 64/ 50/ -10%/ -11% 256/ 1/ -10%/ -17% 256/ 25/ -11%/ -12% 256/ 50/ -9%/ -11% Thanks