Re: [PATCH] kvm: better MWAIT emulation for guests

From	Radim Krčmář <rkrcmar@redhat.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] kvm: better MWAIT emulation for guests
Date	2017-03-13 16:50 +0100
Message-ID	<tkAop-2CJ-7@gated-at.bofh.it> (permalink)
References	<tjeSZ-32y-13@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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@redhat.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

Thread

[PATCH] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-09 23:40 +0100
  Re: [PATCH] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-10 02:00 +0100
    Re: [PATCH] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-10 02:20 +0100
      Re: [PATCH] kvm: better MWAIT emulation for guests Wanpeng Li <kernellwp@gmail.com> - 2017-03-13 08:50 +0100
  Re: [PATCH] kvm: better MWAIT emulation for guests Jim Mattson <jmattson@google.com> - 2017-03-11 00:50 +0100
    Re: [PATCH] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-12 01:10 +0100
      Re: [PATCH] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-12 22:20 +0100
  Re: [PATCH] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-13 16:50 +0100
    Re: [PATCH] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-13 17:10 +0100
      Re: [PATCH] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-13 20:40 +0100
        Re: [PATCH] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-13 21:10 +0100
          Re: [PATCH] kvm: better MWAIT emulation for guests Radim Krčmář <rkrcmar@redhat.com> - 2017-03-13 22:50 +0100
            Re: [PATCH] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-15 19:20 +0100
              Re: [PATCH] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-15 19:40 +0100
                Re: [PATCH] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-15 20:10 +0100
                Re: [PATCH] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-15 20:10 +0100
                Re: [PATCH] kvm: better MWAIT emulation for guests "Gabriel L. Somlo" <gsomlo@gmail.com> - 2017-03-15 20:50 +0100
                Re: [PATCH] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-15 21:20 +0100
                Re: [PATCH] kvm: better MWAIT emulation for guests "Michael S. Tsirkin" <mst@redhat.com> - 2017-03-15 20:50 +0100

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