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 <mst@xxxxxxxxxx> > --- [...] > 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
