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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 13, 2017 at 04:46:20PM +0100, Radim Krčmář wrote:
> 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?

If you are going into a deep sleep state with huge latency you are
better off exiting and paying an extra microsecond latency
since a chance some other task will want to schedule seems higher.

> >                                                           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.

Current linux guests just discover these states based on
CPU model, so we do expose enough info.

>    The feature would need very constrained setup, which is hard to
>    support

Why would it? It works without any tweaking on several boxes
I own.

>  - 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.

Yes I think this limited emulation is safe to enable by default.
Pretending mwait is equivalent to halt maybe isn't.

> What about keeping just the last hunk to improve OS X, for now?
> 
> Thanks.

IMHO if we have a new functionality we are better of creating
some way for guests to discover it is there.
Do we really have to argue about a single bit in HV leaf?
What harm does it do?

> > 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