linux-kernel - Re: [PATCH] kvm: better MWAIT emulation for guests

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Date:   Mon, 13 Mar 2017 16:46:20 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH] kvm: better MWAIT emulation for guests

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@...hat.com>
> ---
  [...]
> 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