Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932303AbdCJAvj (ORCPT ); Thu, 9 Mar 2017 19:51:39 -0500 Received: from mail-qk0-f195.google.com ([209.85.220.195]:35207 "EHLO mail-qk0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754119AbdCJAvg (ORCPT ); Thu, 9 Mar 2017 19:51:36 -0500 Date: Thu, 9 Mar 2017 19:51:27 -0500 From: "Gabriel L. Somlo" To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Paolo Bonzini , Radim =?utf-8?B?S3LEjW3DocWZ?= , 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: <20170310005126.GA19516@foober.ini.cmu.edu> 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-Clacks-Overhead: GNU Terry Pratchett User-Agent: Mutt/1.7.1 (2016-10-04) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4064 Lines: 92 On Fri, Mar 10, 2017 at 12:29:31AM +0200, Michael S. Tsirkin wrote: > 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. Are you getting an IPI if another VCPU writes to the MONITOR-ed memory location? If not, you'd be waking up too late and fail to meet the specified behavior of the MONITOR/MWAIT instruction pair. > 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. 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. Is it a good idea to advertise MWAIT capability to guests? The misbehaving ones will call it willy-nilly, true, but aren't compliant ones better off falling back to some alternative method (typically using a HLT-based idle loop instead of a MONITOR/MWAIT based one) ? Thanks, --Gabriel > > Signed-off-by: Michael S. Tsirkin > --- > Documentation/virtual/kvm/cpuid.txt | 3 +++ > arch/x86/include/uapi/asm/kvm_para.h | 1 + > arch/x86/kvm/cpuid.c | 3 +++ > arch/x86/kvm/vmx.c | 4 ---- > 4 files changed, 7 insertions(+), 4 deletions(-) > > diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt > index 3c65feb..5caa234 100644 > --- a/Documentation/virtual/kvm/cpuid.txt > +++ b/Documentation/virtual/kvm/cpuid.txt > @@ -54,6 +54,9 @@ KVM_FEATURE_PV_UNHALT || 7 || guest checks this feature bit > || || before enabling paravirtualized > || || spinlock support. > ------------------------------------------------------------------------------ > +KVM_FEATURE_MWAIT || 8 || guest can use monitor/mwait > + || || to halt the VCPU. > +------------------------------------------------------------------------------ > KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no guest-side > || || per-cpu warps are expected in > || || kvmclock. > diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h > index cff0bb6..9cc77a7 100644 > --- a/arch/x86/include/uapi/asm/kvm_para.h > +++ b/arch/x86/include/uapi/asm/kvm_para.h > @@ -24,6 +24,7 @@ > #define KVM_FEATURE_STEAL_TIME 5 > #define KVM_FEATURE_PV_EOI 6 > #define KVM_FEATURE_PV_UNHALT 7 > +#define KVM_FEATURE_MWAIT 8 > > /* The last 8 bits are used to indicate how to interpret the flags field > * in pvclock structure. If no bits are set, all flags are ignored. > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > index efde6cc..fe3d292 100644 > --- 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 (sched_info_on()) > entry->eax |= (1 << KVM_FEATURE_STEAL_TIME); > > + if (this_cpu_has(X86_FEATURE_MWAIT)) > + entry->eax = (1 << KVM_FEATURE_MWAIT); > + > entry->ebx = 0; > entry->ecx = 0; > entry->edx = 0; > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c > index 4bfe349..b167aba 100644 > --- 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