Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753594AbdCMPrF (ORCPT ); Mon, 13 Mar 2017 11:47:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39218 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753268AbdCMPq0 (ORCPT ); Mon, 13 Mar 2017 11:46:26 -0400 Date: Mon, 13 Mar 2017 16:46:20 +0100 From: Radim =?utf-8?B?S3LEjW3DocWZ?= To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Paolo Bonzini , Jonathan Corbet , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, kvm@vger.kernel.org, linux-doc@vger.kernel.org Subject: Re: [PATCH] kvm: better MWAIT emulation for guests Message-ID: <20170313154618.GA4547@potion> References: <1489098555-23856-1-git-send-email-mst@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1489098555-23856-1-git-send-email-mst@redhat.com> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Mon, 13 Mar 2017 15:46:25 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2266 Lines: 58 2017-03-10 00:29+0200, Michael S. Tsirkin: > Some guests call mwait without checking the cpu flags. We currently > emulate that as a NOP but on VMX we can do better: let guest stop the > CPU until timer or IPI. CPU will be busy but that isn't any worse than > a NOP emulation. > > Note that mwait within guests is not the same as on real hardware > because you must halt if you want to go deep into sleep. SDM (25.3 CHANGES TO INSTRUCTION BEHAVIOR IN VMX NON-ROOT OPERATION) says that "MWAIT operates normally". What is the reason why MWAIT inside VMX cannot reach the same states as MWAIT outside VMX? > Thus it isn't > a good idea to use the regular MWAIT flag in CPUID for that. Add a flag > in the hypervisor leaf instead. > > Signed-off-by: Michael S. Tsirkin > --- [...] > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > @@ -594,6 +594,9 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function, > + if (this_cpu_has(X86_FEATURE_MWAIT)) > + entry->eax = (1 << KVM_FEATURE_MWAIT); I'd rather not add it as a paravirt feature: - MWAIT requires the software to provide a target state, but we're not doing anything to expose those states. The feature would need very constrained setup, which is hard to support - we've had requests to support MWAIT emulation for Linux and fully emulating MWAIT would be best. MWAIT is not going to enabled by default, of course; it would be targeted at LPAR-like uses of KVM. What about keeping just the last hunk to improve OS X, for now? Thanks. > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c > @@ -3547,13 +3547,9 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf) > CPU_BASED_USE_IO_BITMAPS | > CPU_BASED_MOV_DR_EXITING | > CPU_BASED_USE_TSC_OFFSETING | > - CPU_BASED_MWAIT_EXITING | > - CPU_BASED_MONITOR_EXITING | > CPU_BASED_INVLPG_EXITING | > CPU_BASED_RDPMC_EXITING; > > - printk(KERN_ERR "cleared CPU_BASED_MWAIT_EXITING + CPU_BASED_MONITOR_EXITING\n"); > - > opt = CPU_BASED_TPR_SHADOW | > CPU_BASED_USE_MSR_BITMAPS | > CPU_BASED_ACTIVATE_SECONDARY_CONTROLS; > -- > MST