Re: [PATCH] kvm: better MWAIT emulation for guests
From: Radim KrÄmÃÅ
Date: Mon Mar 13 2017 - 11:47:08 EST
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
- 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?
> 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 |
> - printk(KERN_ERR "cleared CPU_BASED_MWAIT_EXITING + CPU_BASED_MONITOR_EXITING\n");
> opt = CPU_BASED_TPR_SHADOW |
> CPU_BASED_USE_MSR_BITMAPS |